Tagging a Common Event to an accessory

[fgsfds]

Only solution I can give (since Im not a scripter,) is to do a couple of common event with parallel process, then put in some (or alot in your case) conditional branch. If the summon you obtained by storyline, then you can make a switch for that. Say if Yuna doesn't have Behemoth, the switch will be Off, meaning Rikku cannot use it. If Yuna have Behemoth, the switch is On and Rikku will be able to use it.
I don't know much these Aeons since I'm not a FF fangirl but I hope this will point you to the right direction.