You can't do that with any AVR I know of.
If your Spotify source was external to the receiver and both it and the xbox had analog audio outputs you could use an audio mixer to play both into an analog audio input on the receiver.
Altoids Tin 1/8" Stereo Mixer: My car stereo has only one 'Aux' input, but I have several gadgets that I like to hear on a trip without swapping cables: MP3 Player, talking GPS, Blackberry, XM player, Kindle, etc.I couldn't find a commercial solution, so I came up with this lit...
The video output of the Xbox would likely have to go directly into the display rather than the receiver and through that to the display. Most receivers don't make it convenient to change the audio part of an input in the input setup menu.