Question

How does Transform/Imposter work in a Double/Triple Battle?

Comments

And it is clear you just need decent knowledge to understand it

Answer 1

In Double and Triple Battles, you're able to choose Transform's target, provided the target is adjacent to the user. However, Imposter doesn't let you choose — it automatically turns the user into the Pokémon directly opposite it.

Transform (Bulbapedia)
Imposter (Bulbapedia)

Answer 2

Transform: You can chose who to Transform into.
Imposter: It will be randomized what you transform into.

Source: PO tested.