New multi-accounter trick: Paychology games and using GRB from gym offense

We all know GRBs are the #1 gym defender. But some Instinct multi-accounter geniuses are using them for gym offense too now.
So it's basically playing a mind game with the GRB defender: "helping" them feed their Pokemon with an account same colour as the defender, so they will think the person whoever it is will feed for them, then stop feeding and take down the whole gym.

But if they have 2 or more phones, can't they just use the sync attack strategy? And if the defenders feed at 2/3 motivation, they'll probably keep feeding anyway even when they use the "GRB offense trick".