I think you use the Digiface USB as main interface and clock master.
The BBF Pro you can connect via ADAT, you need two TOSLINK cables. The 2nd one towards the BBF Pro is mainly for clock synchronization, so that the BBF Pro as slave can get clock through its ADAT Input. But you can also use it to route audio to the BBF pro, to use e.g. its analog outputs. It might be practical for getting a 2nd headphones output.
The two interfaces use two different drivers; therefore, it is not possible under Windows to use both interfaces at the same time:
- Digiface USB - MADIface driver
- BBF Pro - USB driver
Please note: I would not use 3rd party solutions like ASIO4ALL because then you introduce another ASIO layer on top of the two ASIO drivers. Then you create an unsupported setup. The goal of ASIO drivers is, to have the lowest latencies and stability. If you put a driver layer on top of it, you are weakening this concept, if there is an issue, this cannot be supported as nobody knows whether this is an RME driver or ASIO4ALL issue, and you introduce additional latency.
Regarding stand-alone mode details, please read the manual chapter 30 and 33.2.
I think I would install the two drivers and connect both interfaces, then both are operable via TM FX.
It has the additional advantage, that you do not need a PSU for stand-alone operation (see ch 30).
In the DAW, you only load the driver of the main interface.
BR Ramses - UFX III, 12Mic, XTC, M-1620 Pro D, RayDAT, ADI-2 Pro FS R BE, X10SRi-F, E5-1680v4, Win10