I dont think this will work with a pair of Mk4's (as you have to select between footswitch or external switching on the amp) BUT....on Dual/Triples and Stilettos, the external switching jacks are always active and short tip-to-ground on the channel selected. You can connect the external switching jacks of one amp to the jacks on another amp with some short speaker cables, and connect the supplied footswitch to one amp... changing channels/loop/solo on the footswitch will control both amps. Not sure about Roadster/RK but I would think they would be set up like the other "modern" Mesas.
You can check to see if your amp(s) will work like this by plugging a cable into one of the switching jacks, and changing the channel with the amp's footswitch. Check the free end of the cable with an ohmmeter or continuity tester and see if it shows continuity when that channel is selected, and open when you switch to another channel.
Using a second amp's power section only (as a slave to the first amp) is easier, but doesn't allow for different EQ on the slave amp. It will sound just like the master amp unless you put some type of EQ/FX between the two heads. If this is the road you want to take, IMO you would be better off using a power amp instead of another head...more power, in stereo, for less money.