Stereo pedal will set volume for both amps at the same time. That might be difficult if you switch amps and get a volume drop/raise. He can also use a volume pedal per amp and one master volume in front of AB box. This might also be nice, http://www.singlecoil.com/docs/ab.pdf As simple pedal like that can cost something like 20 dollars if you are a bit handy. Its to easy not to try.