In theory, it should be possible to hear the same bitstream on two devices simultaneously, one bit perfect via a USB DAC and duplicated on your PC internal soundcard to its analog output.
This would be functionally similar to how any low cost set top CD/DVD player sends bit perfect PCM from a CD out its coax/optical SPDIF jack, plus the same audio out its analog RCA outputs.
Using Jack would probably be required to achieve this, perhaps pulseaudio.
http://jackaudio.org/
http://en.wikipedia.org/wiki/PulseAudio
From an abstract point of view, all that is needed is for the bitstream to be duplicated in memory after read/decoded from the source file, then routed to each sound device, a trivial task for a CPU/memory/software, but may be tricky to set up from a practical point of view.
You would set multiple sinks (outputs) from the same source (application/audio daemon/bitstream) without mixing/resampling.
I have never tried this- just thinking out loud.