Many people use good quality power strips from companies like Tripp-Lite because they don't restrict current flow. Keep in mind that even if that duplex outlet were replaced with a quad, you are still connecting everything to one 15A circuit - same as if you picked up a multichannel amp and plugged it into a single outlet.

Outlaw has stated in the past that the M200 is designed to allow up to three amps to be safely stacked together, and I think many people have stacked as many as four (sets of 7 tend to end up in either a stack of 3 and a stack of 4 or a stack of 3 and two stacks of 2). Going beyond around three in a stack is actually more of an issue for structural strength than cooling, I believe - my pair of M200's run quite cool, but they are heavier than they look.

A third thought to help relieve some concern - there's always the 30-day return policy that will let you ship them back for only the cost of shipping if they don't fill your needs. I suspect they will, though - they're great little amps.