In many applications, A simple passive mixer can be used to combine signals. The resistors R1 – R4 are used to reduce interaction of the signals but they do not completely eliminate it. The values may be adjusted higher for more isolation; try 100k for the resistors.
If you add a pot after the input jack of each channel, the volume of the signals in the mix can be easily controlled. Here is the schematic for the DOD 240 mixer that uses this technique.