You can either select the device "A or B" in one TM FX window or open two TM FX windows with device A and device B.
But you can not route audio between the two devices, as there is a routing matrix "per device".
For this you would need to "sacrifice" an ADAT connection to route at least 8ch @single speed or you need to route in the DAW  with the usual  RTL (round trip latency) over USB.
You will need at least one ADAT connection to clock synchronize the two devices, the master will send its clock through ADAT and the clock slave will read it from the ADAT input.
Word clock is no option, these devices do not offer word clock.
If you want to work with sample rates over 48 kHz (double or even quad) then change the protocol of the ADAT port between the two devices for clock synchronization from "ADAT" to "SPDIF", then the clock slave should be able to synch at the proper sample rate, as SPDIF protocol is 2ch with sample rates from 32-192 kHz. With ADAT protocol sample rates over 48kHz are being achieved by port multiplexing, to achieve higher bandwith, but each of the ports stays still at either 32/44.1/48 kHz and a clock slave can not recognize port bundling.
The last thing to pay attention to is, that under Windows you need to check and set the same ASIO buffersize for both devices to get no time offset. With Apple I am not sure, whether there are special settings for buffering compareable to ASIO buffersize "per device".

Do you have an Apple and could use a thunderbolt based expansion chassis? Then perhaps two RayDAT cards could be an alternative. Have no experience though, how good such a HW setup actually works, as I have no Apple here.

RayDAT cards would give you options like:
- internal clock connection or word clock
- 2x MIDI
- AES, SPDIF
- 4x ADAT, ADAT4 switchable to SPDIF

Regarding Protools, there was a channel limit of 32 channels (in/out) in the past, which caused issues with devices like UFX+. This has been increased to at least 64 with 3rd party interfaces.
https://www.synthax.co.uk/latest/2021/0 … -channels/

The Digiface USB has 32ch in / 34 out, the RayDAT 36ch IN / 36 ch out.
If you get two Digiface USB, then it could be the case, that you can't access the last 4 HW Output Channels.
As you might want to set one ADAT port to SPDIF protocol to be able to synch over 48 kHz, then the amount of channels per Digiface USB would be theoretically reduced by 6 channels per device. Maybe by this it would perfectly fit into the 64 channels that protools offers.
The question is, whether e.g. the two phones ports are at the beginning or the end of the 34 channels, in a bad case of luck you wouldn't be able to access the phones outputs of the 2nd device if you would exceed the 64 channels. There is no possibilit to reorder the channels in the driver at least under Windows/ASIO.