A couple ways:
1. If the pedals are stereo, run one channel in each of the amps' loops. You may need to isolate grounds with isolation transformers like the Ebtech HE-2.
2. Use a line mixer. This will let you mix the two FX sends into mono pedals, and then back out to the amp FX returns.
3. If you're using only one amp at a time, there are lots of switching boxes like the GCX, the HGAS and others that will let you switch pedals between multiple amps.