It would be sort of tricky doing something like the 4CM with a two amp setup (really becomes something like the 7 cable method at that point), but I think it could be done. Noise will most likely become an issue, though, and it will need to be addressed in some way.

But from a connection standpoint, it would be pretty straightforward. You'd have to get a TRS to dual mono cable to take the stereo out from the HD500's effects send and take the left side to one amp's front input and the right side to other's. Then you'd take cables from the amps' effects send and back into the HD500's effects return. Finally, you'd take cables from the HD500's outputs to the amps' effects returns.

Where it gets a little tricky is setting up the dual tone in the HD500. You'd have to put the loop before the split, and any of the pre effects you want sent to both amps before the split. Actually, you'd be limited to sending the same pre effects to both amps. After the loop block, you go to the split and then you could put different post effects in each amp's loop.