DIY mixer - use diodes to prevent input potentiometers influencing each other
You need a simple mixer circuit based on the summing amplifier.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Stereo mixer basic schema.

As Brian Drummand says, you need a virtual earth mixer. Here's the basic schematic showing how to handle the mono input.

This diagram does not show power-supply and capacitors. You'll find details in a web search.