Hello Eric,
The ASIO driver architecture only allows a single 'driver' to be used at any one time. In practice, that usually means that only a single computer audio interface can be used at once, although some audio interface manufacturers do provide drivers that have specific functionality to allow multiple identical units to be used together (appearing to the driver as additional channels on the same virtual device), provided that the cards are properly synchronised together in hardware (usually via Word Clock cables).
I don't know off-hand whether RME's RayDAT driver supports more than one RayDAT unit, so I'd recommend contacting RME to check first, before buying one. I see that it requires an additional optional expansion board for Word Clock support (
http://www.rme-audio.de/en/products/hdspe_raydat.php ), so, if the driver does support more than one RayDAT unit, then my guess would be that you would also need one of those for expansion boards for each RayDAT, with appropriate 75-ohm Word Clock cables to connect them together. However, RME would be the best people to contact to verify that for certain.
Another interface that looks interesting for Hauptwerk purposes is MOTU's 16A, which has 16 analogue outputs (
http://motu.com/products/avb/16a ), can connect to the computer via either Thunderbolt, or USB, or AVB, and allows further MOTU AVB units (e.g. more 16A units, or 24Ao units) to be connected/chained via AVB from the first/primary device (e.g. even if connecting the main 16A to the computer via Thunderbolt). A Thunderbolt connection is supposed to be able to handle up to 128 audio channels at 48 kHz, with extremely low latency.
Sound on Sound have a good review of the MOTU AVB series here:
http://www.soundonsound.com/reviews/motu-1248 . The main reported issue with them seems to be that they can be very slow to switch sample rates.
Best regards, Martin.
Hauptwerk software designer/developer, Milan Digital Audio.