Why are these two potentiometers affecting one another?
Yep, as @Andy aka noted, you want to swap your red lines. The result should look like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Here is a pretty good resource: http://www.beavisaudio.com/techpages/Pots/
There is a section on volume control that reiterates the circuit above.