[Aoi Kurenai]

Even before Den-O, Rider would do two parters but they would just be so much more subtle about it - the monster would escape at the end of an episode despite that episode's plot being finished. Or you'd just get the same suit repainted in the next episode.

After a while, Shirakura, the head producer on a lot of the early Rider shows, felt two parters with two Riders should be the new formula of the series. Thankfully, it looks like that's coming to an end.