The RayDAT is a PCIe card with a daughter card connected by ribbon cable to the main card.

The daughter card doesn't need an additional PCIe socket on your main board in terms of electrical connection, but it would be good for stabilizing it in the slot. The daughter card can be placed wherever you want as long as the ribbon cable is long enough.

You can check it on your own if your combination of case and mainboard is feasible to take two RayDAT.

Depending on your other components, you might need the optional WC card, which is also connected by ribbon cable to one of the two main cards.

The two cards need to be clock synchronized and in the driver settings you need to configure the same (ASIO) buffer size..
The clock synchronization can be performed internal, see manual.

The two cards have no internal connection, there is nothing like a backplane, so you have to work with two TM FX instances (either in one or in two separate windows) and route channels from one card to the other through the DAW.
Or you have to provide cross connections between the cards by using ADAT I/O (ADAT offers transfer of 8 ch @single speed).

General information about clock synchronization, see my blog:
https://www.tonstudio-forum.de/blog/ent … ios-en-de/

Yes such information seems to be missing indeed.

But don't worry about this. Audio has not that high throughput demand, even the HDSPe MADI FX with 3x MADI uses PCIe 1.1 x1 according to RME archive site.

PCIe 1.1. So far we did not use anything else.