You will get hum if the amps share the same ground. I read a Guitar player magazine w/ Joe Bonamassa that he stated that he designated 1 main amp ( out of his 4 !
)to have ground and lifted the ground on any additional.
You must be cautious while doing this although, it is dangerous if you're lacking knowledge on this . You will create a ground loop if ground is shared
I am quoting others although.
I run a multi amp setup with a 64' Fender Bandmaster (ground lifted) Dry w/ a Fuzz on a switching system & my 100w Bluesmaster (Wet) , both w/ my main pedal board.
I still encounter slight hum although and have been trying to come up w/ a solution to fix also.