Hi Joe,I have FM9 and FX3 working together, so I can, in theory, have 4 amps.
Actually, using two units can not just sync control switches, but add them up to total 12 of them if you use the CC assignments and external controllers cleverly, which also makes them recordable in a DAW that can record/edit MIDI CC stuff. Expression Pedals need to move out to MIDI to be shared between units, though. Tap tempo works if you "waste" a control switch on it, since you can set the units up to listen for a CC # to get tempo taps. Scenes are also working via the Scene MIDI block, again, with a bit of setup.
Just circling back on this as I have sold off my FM3 but still much prefer using my FM9 over the FX3 (as I just prefer the floorboard config). Since my FX3 is just sitting there unused, I thought that I would try a similar set up where the FM9 ran via SpDIF into the FX3 and then SpDIF out of the FX3 back into the FM9. I would have all pre and post effects in the FM9 with amp/cab or dual amp/cab plus Dyna cabs (plus any always on CPU intensive effects) in the FX3. For some reason, no matter what I try, I cannot get a spDIF signal into the FX3...using all the same setup as I did for the FM3+FM9
Thanks,
Derek