Panning For Fun at Geofex.
A constant level pan-pot is better done this way than with actual pots. It's from a National Semiconductor audio applications book. The circuit expects a low source impedance driving it on both inputs, and inverts phase, so you may need buffers before the two inputs and an inverting opamp stage after it. Two dual opamps make this work even for high source impedances, producing an in-phase result and a constant relative volume pan.