The easier 4CM option would be the FX8 in the AX8 loop. That way you still have XLR outs for FOH.
The more flexible 4CM option is AX8 in the FX8 loop (IIRC, this is the "killer app" mentioned by Matt). You can use FX8 for pre and post effects, but also have pre and post effects in the AX8. If you need XLR to FOH, you'd use a DI after the FX8 outs.
For control, the FX8 is way more flexible. The FX8 sends out a MIDI PC on each preset change on its own channel, corresponding to the preset #. You can also send one MIDI PC per FX8 scene. You also have the 2 MIDI blocks per preset. You can send up to 4 PCs per block and 2 different values per 2 different CCs per block.
In answer to your question, yes you can change presets on the AX8 from the FX8 (just send a PC). You can also change scenes (just send a value of 0-7 to CC #34).
If it were me, I'd only use the AX8 for Amp, Cab, and global effects. Since I don't do a lot of amp switching, I'd only use a few presets on the AX8. Some things like shimmer (MultiDelay into fully wet Reverb) I would do on AX8 because of the flexibility of the grid. Plus, it would make that effect "global" as in it could persist through FX8 preset changes. I wouldn't do beat-divided effects on the AX8 because there's no MIDI clock sync master in this setup. Delays could work if you had an AX8 preset for every FX8 preset, and saved the tempo in both. Gross.
I'd use the FX8 for the per-song meat and potatoes like drives, delays, and reverbs.
Think of it as the AX8 as amp+cab and rack effects, and the FX8 as a pedalboard doing the heavy lifting.
Hmm. I'm tempted to grab an FX8. Doh!