They will not talk to each other.
It is possible to add one to the other as an adat converter to give additional 8 inputs and 8 outputs.
Have one connected to the computer and to the external power supply and do the routing in Totalmix.
All Line-inputs to the adat outputs and all adat-inputs to the Line-outputs. Doubleclick those faders to lift them up to the zero position (fully open). Then in the settings dialog have the clock-mode on internal.
This routing keeps active in the Multiface as long it is powered from the external power supply.
You can disconnect it from the Firewire cable and connect it with the toslink cables to the other Multiface witch should be connected now to the computer and select clock-mode adat optical.
You will be able now to send or receive signals from both boxes in the Totalmix. The adat channels are now sending and receiving from the other box.
Follow this exact and carefully, it will work.
Reupload the same question will not help and video support is likely unrealistic as no one else has a running computer with two working Multiface I guess.
M1-BigSur, Madiface Pro, Digiface USB, Babyface silver