Let’s go back some years on time, at 1958, where a new role was invented: Daini Genga, even if was not credited [Or used, beside of Toei Douga] until the early 2000s, was a crucial part of anime production on these times of Anime, as on this new days, now the term is used when rough gengas caused by lack of time is passed to less experienced animators so they can clean the gengas and convert them on something watchable, on these times, the situation was different, there was only two professional animators on Toei Doga: Yasuji Mori and Akira Daikubara, that were reminiscent of the old and defunct studio Nihon Douga, that later would convert into Toei Doga and then on the modern Toei Animation.
As they were making the transition from Nihon Douga to Toei Douga with only these two animators, they have contracted over thirty animators with almost nothing of actual experience on the medium, with the clear exceptions of Chikao Katsui and Takashi Yamaguchi, both of them that worked on Nihon Douga during a few months before the change, the rest were basically just understanding the basics of animation.
The animation of the first movie of Toei Douga, Hakujaden [That was also Japan’s first animated movie on full color] started on December of 1957, Zenjirou Yamamoto, that was the ex-president of the old Nihon Douga studio, was conferred with the management of the film’s production, and he briefly came to a conclusion to their main problem: Lack of experienced staff. That conclusion was to create “Daini Gengas”, that would clean up the rough gengas of Yasuji Mori and Akira Daikubara, before sending it to the in-betweeners, for that, six of the less unexperienced animators were choosen, Daiichiro Kusube, Shuji Konno and Masatake Kita cleaned up the drawings of Yasuji Mori and Kazuko Nakamura, Chikao Katsui and Yasuo Otsuka cleaned up the drawings of Akira Daikubara [Yeah, Otsuka beginnings were under Akira Daikubara, not under Yasuji Mori, surprise!], each of them were on charge of four or five in-betweeners, and the desk were arranged into six groups, each one corresponding each second, being the leaders of the groups, they get the Genga for the key animators themselves and distribute them between the in-betweeners [No pun intended]. The scenes that the Key Animators were on charge were given according to their style, Yasuji Mori handled the scenes involving animals and the protagonist, that required delicate, nuanced animation, while Daikubara did the scenes with fights and other similar things and required broader and looser animation. Yasuji Mori’s gengas were heavily detailed and nuanced to begin with, leaving little room to improvement, while Daikubara’s one were rough and spare, leaving a lot room to imagination to the seconds and the in-betweeners.
The seconds were assigned scenes also depending of their skill and styles, Chikao Katsui was given the scenes with more energetic movement, Yasuo Otsuka was given the scenes that relied heavily on natural phenomena, Kazuko Nakamura was given with scenes involving Pai Nyan and Shao Chin, Masatake Kita was given the ones with movement heavy scenes, Shuji Konno was given with quiet, delicate scenes and Daiichiro Kusube the other ones, consistently assigning like that the scenes helped on maintaing the overall consistency of the film, even with a complete lack of Sakkan, that was not created until a few years later.
In Hakujaden, after the In-Betweeners finished their assigned shots, the key animators corrected their animation, Akira Daikubara was a lot less strict than Mori, accepting everything as far as the original feeling is maintained, on other hand, Mori never accepted a scene the first time he looked, asking for two or three retakes, after them, he would say “OK”, but he would still secretly fixing them on his desk!, he really wanted the animation to be exactly as i imagined, that style was the legacy that he left to people like Hayao Miyazaki and Yasuo Otsuka, that always were proud of correcting every piece of animation so it would look exactly as they imagined it the first time.
The seconds were never credited as so, instead, they were credited as in-betweeners, only when you key animated a scene is when you convert into a key animator, the exception to that rule was that crazy and rebellious kid of Daiichiro Kusube, after gengas were no coming to him, he leap completely over the hierarchy, and after three months of being hired and being basically a novice without experience on key animation, convinced Yasuji Mori to draw gengas for Hakujaden, without passing from the second stage, drawing the key animation of a few shots fully by himself and passing it to the in-betweeners under his charge, being Kusube the secret third key animator on the movie, without being credited as such, but instead as a in-betweener, doing that for the next two Toei Douga films: Shounen Sarutobi Sasuke  and Saiyuki , being first credited as such in Anju no Zushiomaru , he, then, also permitted to his in-betweeners to draw the key animation for him, on secret, Sadao Tsukioka and Gisaburo Sugii did gengas for Saiyuki, and the same happened with Yoichi Kotabe on Little Prince and the 8-Headed Dragon .
Makoto Nagasawa, who was In-Betweener in Shuuji Konno’s section, starting with the Shounen Sarutobi Sasuke, on 1959, was transferred to Yasuo Otsuka’s section, since he didn’t liked having to draw the incredibly detailed movement of Mori’s cuts, even if he liked the drawings of him, and was more suited to Yasuo Otsuka, also, with Daikubara correcting him, he experimented with the timing of his assigned shots on Saiyuki and injected more personality into the animation of those shots, he debuted as a second on Anju to Zushiomaru on 1961, after drawing his first key animation on Saiyuki the past year, along with Yasuo Otsuka, Daiichiro Kusube, Gisaburo Sugii and Sadao Tsukioka, giving the freedom to everything as they want on the last scene of the film, that had a looser storyboard, he officially became Key Animator in Little Prince and the 8-Headed Dragon, on 1963.
Yasuo Otsuka was the first ascended to the second Toei Doga film, as they brought in two more experienced animators: Masao Kumakawa and Hideo Furusawa, the latter one that was a ex-Nihon Douga animator that abandoned the industry during a while after the disbanding of the studio, having the second film five Key Animators, each of them with one second at charge of their scenes, being then Daiichiro Kusube, Chikao Katsui, Kazuko Nakamura, Shuji Konno and Masatake Kita.
Saiyuki, of 1960, featured the same five animators, along with Osamu Tezuka, with seven seconds assigned this time: Shuji Konno, Masatake Kita, Daiichiro Kusube, Chikao Katsui, Michihiko Yoshida [of the second wave of contracted animators] and Kazuko Nakamura.
Anju to Zushiomaru, of the next year was Daiichiro Kusube’s first appearance as Key Animator, along the other five key animators, the seconds this time were Shuji Konno, Masatake Kita, Reiko Okuyama [Ascended from In-Betweener], Makoto Nagasawa [Ascended from In-Betweener], Chikao Katsui and Michihiko Yoshida.
Sinbad’s Aventure, a year later, featured the debuts of Reiko Okuyama, Masatake Kita and Chikao Katsui to Key Animation, and on the next film at that, premiered on 1963, featured the same Key Animators as Sinbad, but without Akira Daikubara, and instead, replacing him with Makoto Nagasawa, around that time, the second credits became confuse, so i don’t really know if they were there at all, also, since this times, the In-Betweener credits were shorter, the next film only featured 9 In-Betweeners, same with the later ones, leading to two conclusions:
1] They put less effort into the next movies, since they were more busy with TV anime
2] They didn’t need the same amount of in-betweeners to achieve pretty much the same fluidity as the first six films of the studio
It was abandoned shortly after, various staff changes, leading to the beginning of the production of TV anime from the studio, with less importance given to feature films and more staff moving to do TV animation ended up with the system, until it being revitalized on the 2000s by a big number of anime studios, as TV anime’s schedules grew up tighter and tighter and gengas were rougher, Toei Animation and Kyoto Animation are the unique studios that rarely credit 2nd Key Animators, they credit them as “Key Animators”, on a separated group from the other Key Animator, or, as well, they directly didn’t feature 2nd Key Animation at all, being the gengas of enough quality to be send to the in-betweeners directly.
Another thing that leaded up to the temporal disappearance of the role was the long time that they take to access to key animation, which led to Gisaburo Sugii and others to leave the studio for Mushi Pro before they became official key animators, and others just gave up to animation, being incapable of do the same magic as animators like Yasuo Otsuka, Sadao Tsukioka or Daiichiro Kusube.