alright, I think I understand what you want to do here.
there is no magic way to add volume to any head, other than to just turn it up louder.
the way the slave heads work, as with any power amp, is to receive a signal from a pre-amp, and then power it into some extra cabinets. for instance your rig could consist of the mig 100 powering 1 cabinet, and the pre-amp out from the mig going into the coliseum which in turn is powering another cabinet.
from what I can gleam from your syntax I assume you just want to plug them together, turn them both on and hope it gets louder, which I suppose is an admirable quest, but not really what they were made for. hope that clears things up.