AX64 is Thunderbolt, not USB 3.
The way to connect these devices is over MADI, or Dante or ADAT.
One would be the Pro Mon master, the other would be slaved to it clocking off wordclock, or Dante or optical MADI, depending on what you want to do.
Maybe start with what you are trying to achieve?
In this thread and the previous one you’ve asked a few question without really being clear about what the use case is.
Do you simply want to add Thunderbolt to your studio?
In that case just wait for the TB3 card for the MTRX studio to come out.
They are, apparently, imminent.
But to your question, if you have two Avid/DAD Pro Mon capable products and are running a single control room then it would be usual to make one the master and one the slave unit.
As I said in the other thread you cannot have two devices being the Pro Mon monitor controller on one computer.
You designate one as the master and the other as the slave.
The slave clocks off the master.
With the AX64 and the MTRX Studio would make the AX64 the master (as it has a way bigger routing matrix) and serve up the MTRX Studio’s IO and Digilink channels to the AX64.
This is what I was previously doing with the AX64 and the MTRX I.
Now I have a MTRX II and an AX64 I designated the MTRX II as the master and the AX64’s IO is served up to the MTRX II using DADlink, but you could use Dante or MADI, or even ADAT if you wanted to.
Does this all make sense?