first: If you are plugging your dmg's into mono channels you need to pan one of them left and the other right.
second: "to" and "two" are two different words. Slow down when typing/posting a thread.
Now... Explain your entire setup going from the output of the gameboy to the amps/speakers.
if you take the cable that infradead posted a picture of out of your dmg and into either (2) Mono channels of your mixer or (1) Stereo channel of your mixer
it should be pretty straight forward if you have stereo channels.... but with mono you have to remember it's taking a mono signal and sending that out both the L and R 's of your mixer. so you have to "assign" them to Left and Right via your pan knob on the mono channels. So you'd take your Left input from the gameboy and pan it Left on the mixer. and your right input from your gameboy and pan it right on your mixer. that should split the two signals to L and R of your main outputs.
Good luck in your signal flow adventures! It can be a head twister at first but once you understand the inner workings of how mixers and signal flow works
it really gets easier. If you have a block diagram of your mixer you should take a look at it.. it will probably help make things understand.
here is an example of what i am talking about.