so, after watching and finishing build, i decided to watch the beginning of Heisei phase 2, Kamen Rider W, what i didn't expect, was that Kamen Rider Accel would be the second rider, since he and Double don't really share a motif (atleast at first glance), i thought accel was from another series, in fact, i actually thought he was the main protagonist of it's own, bike/vehicle themed Rider season.
also, going back to secondary riders, before watching OOO (my first ever rider) i thought that the main difference between Kamen Rider and Super Sentai was that the Riders where always fighting solo, instead of having a team like the rangers, i didn't know secondary Riders existed, i just thought that each series had only one Rider that would stay alone for the entire season, then i watched OOO, and got introduced to Kamen Rider Build, the season's second rider, and after going around researching a bit, i found that having two riders or more per season was actually a tradition of the franchise, since basically it's creation with the original 1971 series... so yeah...