You need something to act as a load for the Mark IV power section so you don't blow the transformer. Power sections need to run to a speaker or speaker like device (attenuator than can fully absorb the load then have a line out). Then you can run the preamp sections of both amps through a RG-16 or Effect Gizmo (made by RJM, Boogie Board sponsor) or similar hardware, and have the out at the end of the switcher go into the effects return on your LSC. If you want to get really crazy, get a Mesa High Gain Amp switcher and you can switch between the LSC and Mark IV power sections too and won't need a separate load.
So your signal would go like this.
Guitar directly into RG-16 or Effect Gizmo "in" jack
Loop 1 - BB Preamp
Loop 2 - Mark IV preamp section only (send from RG-16 into the input on Mark IV, effects send from Mark IV to return on RG-16, some kind of load connected to the Mark IV power section)
Loop 3 - LSC preamp section only (send from RG-16 into the input on LSC, effects send from LSC to return on RG-16)
Loop 4 - EHX Memory Man
RG-16 or Effect Gizmo out to the Effects return of the LSC.
If the EHX can't handle line level then it would go in Loop 2 and push the preamps to loops 3 and 4.
Good luck. Sounds like its going to be a very cool rig.
Bobby