What will you be driving the other side of the cabinet with? Your Mark IV isn't a stereo amplifier so you'll only be going to one side. If it has Vintage 30s in it you also run the risk of blowing the one speaker that would be active while running the cab. If you have two amps You're not going to get much stereo separation from a 2x12 anyway. Kind of seems like a waste of time to me. Stereo effects need separation to sound their best so you're better off just getting another one down the road along with a Simul-Satellite to run it. Just my two cents. Take it or leave it.