What do you mean exactly? The two interfaces connected via USB or one connected to the the other through ADAT?
Both is possible. Both use the same driver. Also stand-alone mode is possible.
When connecting / operating both in parallel you need to clock sync them through ADAT.
And you need to ensure to set for both interfaces the same ASIO buffersize (if Windows).
As the converter have a little difference in terms of conversion time you will notice a little (minor) difference in timing on recorded tracks which you should be able to compensate in the DAW.
BBF            converter conversion latency @44.1: A/D 43 samples 0.98ms, D/A 28 samples 0.63ms
BBF Pro FS converter conversion latency @44.1: A/D 5 samples   0.11ms, D/A  7 samples 0.16ms

RME tests up to three devices for their drivers, theoretically even more are possible, but not tested.
In terms of clock synch you can chain it through ADAT.

PC------USB------BBF1                            clock master
                           |
                          ADAT OUT
                          ADAT IN
                           |
PC------USB------BBF2                            clock slave, gets clock via ADAT IN from BBF1
                           |
                          ADAT OUT
                          ADAT IN
                           |
PC------USB------BBF3                           clock slave, gets clock via ADAT IN from BBF2

I showed here only the TOSLINKs important for clock synch, of course you should also connect TOSLINK cables for the other transfer direction for audio.

Each interface has a TM FX instance of it's own. So you can't simply route e.g. a HW input of BBF2 to HW output of BBF1 in terms of the digital routing matrix of each device, this is a fix boundary. But you can of course route up to 8 channels through ADAT at single speed or 4 channels at double speed, etc.