Birth by Sleep Trinity limit

Me and my brother both have BBS so we can do the joint struggle together, but we always hit a problem. My favorate limit is the Trinity Limit but when ever I use it with my brother he is ether unable to join the attack or worse his game crashes.... We have only managed to use it together once, does anyone know why this keeps happening?

 

Maybe there's something wrong with his PSP. Could be that using the trinity limit puts too much stress on the system (or he doesn't have enough storage space on his memory card and needs to get a new one for a giant data install) and it crashes or it just refuses to do it.

 

Thats what I thought but I got him a new memory card with a much larger memory than before and it still goes wrong...

 

Then it's very likely that it's either his copy of the game or his system.

 

I don't know.... I'm not sure it is his system OR game...