You also could use FF UC/UCX (maybe also  FF 400, I have not checked),  if you do not mind to have two chanels unbalanced, or if 6 ballanced would do.

If using USB (not for FF 400) you could also attach it to computer directly and you would have 8 more analog channels even on 96k.  The only drawback would be that the would be independent in TotalMix. But still, you could route ADAT channels also to UCXII, and have them in TM (either all 8 at 44/48k or selected 4 in 96k).

DAW would see the both interfaces as one.

Be aware you have to configure the FF800 first before you can use it as a standalone converter. So you will need the firewire connection to a computer at least once!
It’s

FF UC or UCX? It may be even cheaper than FF 800.

Audient EVO SP8 - on the budget side, but pretty decent. I guess it can bypass mic pres.

Btw this topic is the very reason I miss OctaMic XTCs a lot, with their ins on the back.

When you use the FF800 as standalone converter the buffersize in the computewr will not affect the FF800.
The routing is also to do in the other computer in Totalmix, route every Adat input to an analog output and every analog input to an Adat output.
Save setting into device.
Then it is ready.
Clock-mode external Adat optical.

With 8 short XLR cables the front inputs could get accessable from back side https://www.thomann.de/de/pro_snake_tpm_05.htm
The rack just need a small space of the thikness of the cable, fix installation

Yes, you will need to separately flash the Totalmix settings/routings (in the Options menu IIRC).

Yes, there are always separate Totalmix windows for each device. Totalmix runs on the interface hardware (DSP) itself.

After setting it up properly, the second interface (UC) in standalone mode acts like every other ADAT AD/DA converter. It’s not 'seen' in Totalmix. There you always have the ADAT channels, regardless if such hardware is connected to the main interface or not.

ADAT carries the clock signal (in same direction as audio signal), no need for a word clock cable.

Some additions to this topic:

[Windows] regarding UC and UCX II in parallel, possible as they use the same driver.

Note: as the converter latency differs between the two units, expect a little time offset in tracks:
UC:     43.2/28   (AD/DA) samples = 0.98/0.63ms @44.1 kHz
UCX II:  5  /  5.8 (AD/DA) samples= 0.11/0.13ms @44.1 kHz
So, when recording, 43.2-5 = 38.2 samples = 0.87ms @44.1 kHz

When connecting stereo devices to your setup via analog ports, then take care, to use ports of the same device; otherwise you would get this offset between stereo channels (to be avoided).

[Windows/ASIO] during operation you have to ensure in the driver settings, that both units are set to use the same ASIO buffer size otherwise these offsets will become much higher (as the latency over USB is (and can become much) higher compared to converter latency).

UCX II with the FS should be clock master as it has the more precise clock.

For clock synchronization, I would use the coax. SPDIF port instead of the ADAT port for two reasons:
1. This leaves the ADAT ports of both units free to connect other gear in the future, be it preamps or AD/DA converter.
2. SPDIF (2ch protocol) allows for sample rates up to 192 kHz, where the other side can clearly detect the clock.
With ADAT this is not possible, as the higher bandwidth demands for double and quad speed are delivered by channel multiplexing, not by using a higher clock rate. When ADAT protocol with channel multiplexing is in use, the clock slave can't detect whether this is 44.1, 88.2 or 176.4 kHz. Its possible to use, but requires manual adjustments, the clock won't follow automatically.

Operation of TM FX: you can choose between opening two TM FX Windows, one for each unit.
The alternative is to switch between UCX II and UC.
In the upper-right area, there is a pull down menu where you can select the unit (UCX II or UC).

[And if you want to get a little more out of the setup...]

DURec as backup recording

If you want to use DURec as backup recording then this is also possible for a few channels of the UC when connecting the UC via ADAT and coax. SPDIF.
Here the max amount of channels which are possible to transfer depending on the sample rate being in use:

10 ch (8 via ADAT + 2 via coax SPDIF) @single speed
  6 ch (4 via ADAT + 2 via coax SPDIF) @double speed
  4 ch (2 via ADAT + 2 via coax SPDIF) @quad speed

                                                PC
                                                  | USB
                   +----------------------------------------------------+
                   |                                                                   |
               [master]                                                       [slave]
               UCX II---SPDIF(c)-----clock sync--->SPDIF(c)---UC
For DURec :    \                                                               /
                       \---ADAT IN<-----8/4/2ch-----ADAT OUT---/
                        \---SPDIF(c)<-----2ch---------SPDIF(c)---/