Hmm...

Well, there's honestly not much you can do to...fix that in PvP situations, unless there is a way to have the players send out their pokemon at the same time.

A collab might be the best solution to fix that, but I'm not sure how everyone here feels about them.

The two battling could also maybe decide what pokemon they would send out first in the OOC, and come to a compromise or decision that their character would send out 'x' pokemon and the other would send out 'y'. Relies on the honor system a bit, but I tend to give people the benefit of the doubt in such situations. As long as it's reasonable that their character would chose said pokemon first for a battle, I don't see the problem with it.