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How to mix stereo & mono inputs for XBox headsets

Does anyone know the best way to merge/mix a stereo input and a mono input so that the stereo remains stereo, but the mono input can be heard across both ear pieces?

I've got a project in mind to create a box which will do this for using normal headphones for my Xbox 360 in-game chat.
I'm just not sure if I need to isolate the mono input with a diode so that it doesn't turn the stereo input into mono.