You are looking for a mixer by the sound of things.
You want 2 separate analog outputs to be combined and then passed onto an input on a amplifier.
If you are the DIY type one(inverting) or 2(inverting then un-inverting) dual channel opamps and a handfull of resistors can do the trick. This basic setup would rely on the volume control within each device instead, but it could be added if needed.
Outside of the power setup, this is what a 2 channel setup would look like(it is a crop from a 6 channel version, but it is just the same thing 3 times in a row.). Non of the capacitors are required, but better safe than sorry.