Mix and match an R30iB Plus control with an R30iB control in dual arm?

When you dual arm, 1 controller has the CPU removed. I think the only thing different at that point would be the axis control card. You should ask Fanuc to confirm if that's an issue.

Or consider if you really need dual arm control vs 2 robots working together with their own controllers kept separate.

Your controllers are most likely capped to control six axis, but you can confirm with Fanuc with your robot's information.

I know you can control more than one robot with the same controller and pendant, but that requires you to purchase and option and load it to the controller and also to purchase a card that is capable of more axis and replace the one inside your controller. As long as you are staying within the same family of robots, if you try to go from a 100i to say a SCARA.. you will need a additional controller