Oh, and I forgot to mention. There's a preamp out there that has the exact setup you're asking for re: mid/mid sweep & bass/treble on stacked pots.
The
EMG BQC control. Basically, the output of your vol/vol pot would go into this pre, and then out to the output jack.
I've had one of these in my bass for 8 years now. (technically the EMG "BQC system" which is just the "control" with a blend pot that's geared for EMG's active pickups. Clip off the blend pot and you've got the "control") I'm going to be removing it soon. Decent preamp. The EMG pre isn't going to have the same sound as the Aggie though.