When AA 3 opens an ASIO device, it hogs takes possession of the whole device, even though some device drivers are built for multi-client support. Other audio applications that allow for this functionality have preference screens built in with which you can choose the individual channels that will be used by the software - if a series of channels is not in use by that one application, then another audio application can utilize the unused ports.
For example, I may choose to activate and use digital output channels 1-8 of an RME FireFace in Sonar as VSTi outputs, do a loopback in the RME TotalMix mixer, and then choose to use those same inputs to record into Reaper. As long as I have only activated the 8 outputs in Sonar and the 8 inputs in Reaper, everything is copacetic.
One can also use the pairs of outputs and split these multi-client interfaces into two halves (or any division, really) and have quite a bit of versatility - the only caveat being that there can only be a single word-clock (sample rate) for everything.
If the developers could see fit to implement this in the next version, it would be nice. I would like to get down to using just Reaper and AA and this recognition of multi-client capability would certainly help that cause.
With CoolEdit I could use the RME Digi96 soundcard together with Digicheck.
But with Au I can not. A pity...