No, it will just work.

I have a solution for you, here we go ;-)

Before doing any changes, save your current TM FX configuration:
- If not yet done save current routing to a same or new snapshot (1..8)
- File -> Save Snapshot as
- File -> Save Workspace as
Populate one of the 30 "Workspace quick select" Slots, which saves everything (snapshot and workspace), because from there it's easiest to recover without having to recall it from single files on your computer.
You can i.e. use the last Slot 30 for Backup Purposes, call it "Backup1"

In TM FX:
1. Assign Phones 3 to M49/50 (from 2nd last group of 8 MADI channels)
2. Assign Phones 4 to M51/52 (from last group of 8 MADI channels)
3. Ensure you have activated "submix mode" (TM FX right side under "View Options")
4. click to HW Output M49/50 and create submix for "Phones 3"
5. click to HW Output M51/52 and create submix for "Phones 4"

Save some work
In regards to step 4 and 5 you can safe eventually some work. If the submix of Phones 3 and 4 is similar or equal to your Main Out submix (UFX+ AES) then see UFX+ handbook how to
- either "copy a submix" (chapter 27.2)
- or to "mirror a submix" (chapter 27.3)
which is a time saver, especially if your submix contains a few channels.

In particular:

• A copy of a submix allows you to make changes at any time to each individual submix and gives you most flexibility, but also the little operational burden of having to perform routing changes like the addition of channels to a submix individually.

• This work you can save by creating a mirror.

Lets put an example
Lets assume UFX+ "Main Out" contains the perfect routing also for "Phones 1-4"

• "Main Out" is mapped to UFX+ HW Output "AES Out".

• Your "Phones 1-4" channels are mapped to "Analog 9/10 Out", "Analog 11/12 Out", "M49/50", "M51/52"

Steps:

Copy / Mirror
6. TM Control Room: right-click output channel "Main Out" (AES Out)
    from pull-down menue select "Copy/Mirror Output Main"

Mirror submix to these channels:
7. TM Control Room: right-click output channel "Phones 1" (Analog 9/10 Out)
    from pull-down menue select "Mirror of Output Main"
8. TM Control Room: right-click output channel "Phones 2" (Analog 11/12 Out)
    from pull-down menue select "Mirror of Output Main"
9. TM Control Room: right-click output channel "Phones 3" (M49/50 Out)
    from pull-down menue select "Mirror of Output Main"
10. TM Control Room: right-click output channel "Phones 4" (M51/52 Out)
    from pull-down menue select "Mirror of Output Main"

Routing to phones on Octamic XTC:

11. Push Phones Button on front, menue changes to "Phones 1"
      Turn Encoder 2 to select the source for Phones 1: MA 49/50
12. Push Phones Button again, menue changes to "Phones 2"
      Turn Encoder 2 to select the source for Phones 2: MA 51/52

Further operational tasks

No matter whether you create the submix
- manually
- by copying
- by mirror
you can individually move the fader of Phones 1-4 like you require, mute individual outputs ...
And save the settings digitally in snapshots of the UFX+.

Save the new TM FX config of your UFX+
- If not yet done save current routing to a same or new snapshot (1..8)
- File -> Save Snapshot as
- File -> Save Workspace as
Populate one of the 30 "Workspace quick select" Slots.
You can i.e. use the next free Slot (1..29, 30 is your backup), call it i.e. "New Config using Phones of XTC"

Save the config also on the XTC
The new configuration of the XTC I would also save:
- Push Setup Button in the front
- Turn Encoder 1 to the right, then you are in the Setups menue
- Save the config to one of the configuration slots 1..6.

Optional next steps
If not yet done already, you could beautify TM FX by adding/removing channels from the view that you do not require
TM FX -> Channel Layout TABs "Input", "Playbacks", "Outputs"
6 different layouts you can store and recall from Layout Presets on the right.

Thats it, have fun !

TM FX is the best and most flexible mixer for recording interfaces, amen