Here's my thought on how I'd do it.....
I'd wire up a dpdt (double pole double throw) switch between the inout wires from the "on" posts of the pickup switches, then wire those up to a "blend pot, which would be a double decker control potentiometer, the middle wire would go to the output tab of the 2nd switch.
It'd be easier to just show a diagram, this is how I'd do it at least...
The reason for the DPDT switch would be to bypass the direct on/off signals, as the tone is even slightly different using a blender. Also, if cleverly worked out, one could make it where it's easily removable should you want to sell/change the guitar/bass back to stock.
