Why run two amps/cabs?, just have a line selector (Boss, SFX, COG, Lehle etc) that have two channels into one output.
The Boss does not allow you to run each channel independently as well as both together (on one setting anyway), which is a shame. My SFX does, and COG have built a similar one to that which also includes phase adjustment.
All of those mix to a single output, meaning you have two channels into one amp. If you want something with an output for each channel (so into two amps), some further research you may have to undertake*
Si
*went a bit Yoda there