Are you bothered about changing leads?
If not you could do it really simply with one box. It'll have one footswitch, the switch just engages an automatic (or set with a knob) drop. On true bypass it does nothing, on stomping you lose the extra by the amount you've set, obviously it still requires lead changes and to remember which instrument is in your hands in reference to the box being on or not. But it's very simple.
If you want both basses plugged in all the time, an AB box with level controls on just one of the input channels would do the trick, just plug the active into the input with the knob, the passive into B, and set the active to be the same level. Then just stomp when you switch instruments.
No need to make everything too complex. There's probably some commercial pedals that do it, or at the very least, there will be circuit diagrams available for a spot of DIY.