One one interface at once can be the Pro Mon monitor controller.
You can switch from one to the other easily but you don’t share Pro Mon between two devices.
The way you do it is to designate one device as ‘master’, make that your Pro Mon monitor controller, then serve IO from the second device up over MADI/Dante/DADlink (or AES, I guess).
I can’t think of a way to combine TB Channel count from multiple units without doing it via aggregate in Mac OS, which I wouldn’t bother with.
But it is easy to serve the slave unit IO up to the primary interface using DADlink- this is what I am doing.
The MTRX II is the master, the AX64 serves up its IO to the MTRX II over DADlink.