Hmm, sounds like it'd be worth a try. Especially with Fourze's Hyper Battle DVD and all.
For the time being, I've tried certain teams (for the like less than ten characters I've unlocked), and:
-Fourze + Meteor: Rider Rocket Drill Kick + Elec Meteor Strike.
-Double + OOO: As said earlier (and seen on the site, apparently) the Double Gold Xtreme + Tajadol Kick combo of the Core movie.
-Ryuki + Knight: Both perform their finishers after the other. No combination finisher, sadly.
-Black + Black RX: Both perform their finishers after each other. No combination finisher.
-Ichigo + Nigo: Rider Double Kick.
-Faiz + Kaixa: Both perform their finishers after each other. No combination finisher.
-Den-O + Zeronos: Both perform their finishers after each other. No combination finisher.
-V3 + Riderman: Both perform their finishers after each other. No combination finisher.
-Double + Accel: Both perform their finishers after each other. No combination finisher.
-OOO and Fourze have no combination.
Also, someone totally should translate the items in Yuuki's shop. Some look pretty interesting (like those L + X + O ones, I'm guessing they're new finishers? Going to buy one to try.)