This is should be possible. However, this application is very specific and we cannot offer any direct instructions. We did find this discussion on the same topic: https://bbs.archlinux.org/viewtopic.php?id=64567
You could also buy an input splitter with two inputs for your output. Or, if you still have a digital input left, you could buy a HiFiBerry Digi+ to connect your Pi and free up the AUX input.