Am I correct in presuming you're looking to use the preamp side of one particular amp and then run it through some effects and then back to the power amp stage of several different amps?
Something like a Voodoo Labs GCX would probably do anything you want in terms of that sort of routing.
Assuming you've got one chain of effects in the loop, then you are looking for a box that does ab/y into the chain and then ab/y out of the chain. The GCX would handle the ab part at either end but you'd probably need add a little mixer to get the y part at the front and a splitter to get it at the return to the two amps.
Logically you'd need something like this. The GCX could handle the switching part but you'd need a mini mixer to combine the two signals coming in and a splitter box (or even just a cable) at the end. Biggest issue will probably be ground loops though.
Just spotted this little box on the Sound Sculpture website. If you're not adverse to a bit of Midi, two of these would probably do exactly what you want.
"If you've been looking for a way to include AB Switching in your midi controlled guitar, keyboard, or effects rig, look no further. The ABCadabra (pronounced "A/B Cadabra") is a midi-controlled relay-operated audio AB switch which can be used to connect your instrument to either of 2 amplifiers or to both amps together. The device can alternately be used as a true bypass mono effects loop, via a stereo to dual-mono cable. In this mode it allows the use of midi to insert or bypass an effect allowing direct connection between your guitar and amp when the effect is bypassed. The ABCadabra can also be used to select among two signal processors into a single amp when connected in reverse. "