I do not own the Basswitch personally, but looking at the pics I am guessing that both channels are affected by the same volume knob at the input stage.
If your electric and upright have the same levels, i.e electric Pickup and the Realist, you should be able to use the Boost to get your piezo up to matching levels.
For doubling, I use the Radial Bassbone (older version). It has 2 separate volumes (cut only, it does not boost the signal) for each channel and I can use the boost (switchable) to match up my volumes or give me some extra volume for soloing if needed.