I've got a RME 9632, a Presonus FS ADAT Out to RME ADAT In, and am considering a Apollo Twin USB3. I have only used the ADAT inputs to record multiple analog signals at one time. Can I select  the UAD for one track, the Presonus for a 2nd track (or more), and record all simultaneously? Thanks, Shep

No.

ASIO drivers can only address one hardware unit at a time.

ADAT is different in that the driver can see ADAT inputs and outputs.

However devices from different manufacturers don't usually play nice together. You can do a whole load of experimenting to see if you've stumbled across just the correct combination that might work but it's not likely. You still need to synchronise the sample rate clocks between the devices.

On the other hand, using WDM drivers will allow you to address multiple devices but then you still need a way to synchronise the sample rate clocks of all the involved devices. One must be a master and the others slaves. At the very least you'll need SPDIF inputs and outputs to connect to pass the synchronising signal or proper clock IO.

So let's see if I follow... Since the UAD Twin USB has no SPDIF IO to lock up with, (RME and Presonus are ADAT SPDIF locked) it has no chance of being tight to the other clock bound devices - even if I find a way to get both to work simultaneously. It makes me wonder why such a fine piece of gear would omit such a indispensable component of recording.

Using Spdif as a synch connection is only a workaround. It works because Spdif carries a synchronising signal along with the digital audio. Spdif is a consumer grade connection so not always found on high end gear.

And because units from different manufacturers don't work together there's not really a need. Even with the same manufacturer there's no guarantee that their units will cooperate. It requires the driver to be written to accomodate the function and even hardware clock IO depending on how thorough the driver implementation is.

It's a limitation of Windows and how the ASIO drivers are written. Macs allow you to aggregate drivers and present them as a single unified while to the software.

ADAT carries its own clock signal. RME usually has clock IO specifically but then RME drivers are usually written to allow a number of same devices to be aggregated.

The extra work in the drivers and hardware drives the cost up considerably.