Im hoping hippy will come along and give advice here... but the idea I had was two stepped attenuators each with..its own attenuation control..... and then you could keep them both on maximum, and reduce the attenuation of the channel that you want to be quieter.
The wonderful thing with the type of attenuator shown below is that you can vary the resistors in place according to what you want to do - for example you could go all the way from no resistance (virtually) all the way to complete silence... and you should maintain a purer signal than by going through a more conventional balance control by virtue of the fact that the signal has only passed through two resisitors.
NB If this is a method that others reccomend, on another thread on this forum I think John is about to bulk buy some goldpoint stepped attenuators (reputedly the best kind) for a discounted price.