I'm not familiar with the RME but it looks a lot like the MOTU Ultralite.
The Ultralite has a built-in mixer that you can control from the front
panel as well as a software virtual mixer. Using this mixer you can route
any inputs to any outputs, RME probably has the same thing.
If you can't do this in the hardware, then you'll have to mix them
in software. This will require running Mobius as a VST plugin,
then mixing in the host. For Bidule you would just connect
input pins 1 and 3 from the RME to Mobius pin 1 and RME pins
2 and 4 to Moibus pin 2.