A couple weeks ago I wanted to use both of my combos. So, I bought an XLR to 1/4" cable. I made the mistake of ordering an XLR to 1/4" stereo cable. There was so much hiss when I linked my amps that the cable was useless.
I have 2 multi-effects pedals, a Digitech BP90 and Zoom B3. Both pedals have stereo 1/4" outputs. Tonight, I ran both of my combos using my B3. It worked great. Unless both amps are identical, you will DEFINITELY need to tweak the volume and clean/vanilla sound of each amp so that they match.
Basses: Squier VM Jazz TSB - Squier VM Jazz Natural - SBMM SUB ray5 - SBMM SUB ray4
Amps: TC Electronic BG250-210 - Hartke Kickback 10
Pedals: Zoom B3 - Digitech BP90