[Librarian]

I'm going to take a random guess entirely here.

Zi-O is the lead rider. Secondary is the tutor for Zi-O but is only 8-9 years older.

Not brother and sister, but sometimes do act like it. The older one tends to be more background though wanting Zi-O to take center stage but may have a tendency to act all know it all, but Zi-O is as smart, and catches onto things the other misses in details, might even be smarter and is definitely heavily sarcastic and for lack of a better word, a little sassy(in a fun way!). And it's likely because of the secondary that Zi-O gets their change device and is immersed in rider history/lore. But it's something they both enjoy.

May have a growing romance angle, but may not start that way. They are moderately similar with a few minor differences due to age/experience and how that alters their perceptions. Both have attitude and are a bit "deadpool" like, but from different angles.

That's my random theory.
Partially based on the Stronger/Tackle dynamic if we're going by her 60th birthday.

Or new theory.
One belt/henshin device but two different people use it at different times. The poses seem too different to be the same. And the suit actors seem different too. Which if it's only one rider with "AM" and "PM" forms, maybe AM is one wearer, and PM is a different wearer and they trade who wears it for some reason. If it was the same person, you'd think the hand gestures wouldn't be so wildly different to convey such different attitudes. And it would be an interesting inversion of the W motif of two in one. Fits the time theme too. One would be a morning person, and the other would be a night owl/up late kind of person. This would fit sibling(adopted maybe?) theory too, or potentially live together for some reason. Or maybe even something kind of like "Last Hope" with the 'found family' aspect of raising each other.