1) The UFX is not designed for this type of use, and it is not supported by RME.
2) PT will only see the first 32 I/O channels, so this does not work with PT if each unit is connected to the computer directly.
3) The second UFX can be used in AD/DA mode, connected to the current UFX via ADATs. Then PT can see all the analog I/O of the 2 units.

Yes, only 8 ch via ADAT at 96k.

I use two UFX with one unit as a slave. That results in a max track count of 22 @ 24/96. I also can record up to 6 "Bonus Tracks" with DURec off the front of the UFX slave. That's because unused on the slave UFX there are: 2 line inputs, AES and 1 ADAT that do not go to the the master UFX. But this requires importing these DURec tracks into your DAW session sometime later.

Something like a Fireface 400 would be useful to provide 4 more converters (for AES and Smuxed ADAT) and 2 more mic and 2 line inputs. You would, additionally, need a quad mic pre if you require all 6 DURec tracks as mic inputs. This results in 28 tracks of 24/96 with the extra after session work of importing the DURec tracks.

DURec is most useful for me as an expedient to record infrequent solo performers in a live session. And if that tune or take is not a keeper the Bonus tracks need not be imported. For a higher than 22 24/96 track count the equipment costs really escalate.

Also realize that, depending on your hardware (I have just installed a SSD in my 2011 17" MacBook Pro in hopes of improved PT 10 performance) and software settings, ProTools 10 can be somewhat buggy with 22 24/96. More so than PT 9 in my experience.