Some scattered thoughts.

Are you trying to do this playback through Pro Tools? Pro Tools will see one hardware item. For example, I have GT-100, 11R and Mbox hooked up via USB. All my audio gets in to and out of Pro Tools via the selected hardware's drivers. The "selected hardware" is what I've selected in Pro Tools. In my case, that's the Mbox.

ASIO4ALL is indeed one of the long time standards. It's free. It's also finicky and will not work for no apparent reason sometimes. It also may solve your problem. It may very well be on your machine already from other things.

One possibility is to try to "activate" your interface's output via the sound controls gotten through Control Panel. I've had both monitors and my decent PC speakers running at the same time quite by accident, which is how I discovered that it could be done sometimes.

Does your interface have a SPIDF connection? I think there's a 10meter maximum preference, but it's digital data. It may still function at 20 feet. A roll of the dice to try in terns of buying extra long cables and possibly connectors. That would llow you to run 11R thru both USB and your interface. You can pickup the 11R via the interface from inside of Pro Tools as one of that interface's channels. It will also send output to the selected hardware device (your interface). If you already have short SPIDF cables to test with, you could temporarily move the 11R closer to your interface.

This should not be an issue as PT would have let you know already, but make sure your interface and 11R are running the same speed. If one's doing 48Khz, make the other one 48Khz.

Good luck!