Seconded. Supervised out of cage time together might still be OK.
It doesn't really change anything but I did want to say that parrot social hierarchies don't involve dominance. The parrot that is doing the plucking might be frustrated for lack of destructible toys, hormonal, or just plain rude, but he probably isn't trying to be dominant over anyone. I still think you should separate them but maybe add more toys to the cage of different types too. Toys with little shreddy paper bits might resemble feathers he can pluck! Lol.
This video is about height dominance, which is something different entirely, BUT she does address the myth of dominance in birds in a really nice way, so I thought I'd link it: