Hey, I'd think if you get that hum issue worked out those 2 would make a great pairing/stack!
Does either of your combos ever have hum issues while used on its own?
I rehearsed/gigged my stacked B100R pair quite a few times and never had any Hum/noise issues. I'd started out linking mine with amp-A's pre-out into amp-B's input, and then settled on using an old Morley ABY pedal into each amps inputs, again never had any hum issues with either connection method. I dont think I ever actualy tried using a Y-cord to link my pair.
I was thinking If you were connecting Y-cord/adaptor between your bass and each amp that mighta been causing your hum. (either that or meyby one of your guitar cords was actualy a speaker cable, I've made that mistake myself on rushed load-ins/dark stages...)
Were both your combos plugged into the same 110v. power outlet? If not, meyby there was some weired issue with one of them causing your hum.