An a/b switch won't let you run both at the same time. An a/b/y switch will let you either switch between one or the other, or have both on at the same time.
I've done basically what you want to do with one of
these. I had that aby pedal at the beginning of my pedal board. I ran one channel through my effects pedals into a guitar preamp, then into one side of a stereo power amp, then into a 2x10 cabinet. I ran the other channel into a bass preamp, the other side of the power amp, and into a 2x15 cabinet. Even with a cheap, s***y DOD Grunge pedal I got great distortion tones.
There are a lot of options out there. I can say from experience that Loop-master makes great, high-quality stuff at really reasonable prices.
-Y