Q switches don’t switch between mono and stereo. Rather, the Q switch, either 2-way or 3-way, provides a boost at the cut-off frequency set by the filter. As a guitar player, I am less familiar with the Brown bass than some other members, but I believe, like most other non-Series Alembics, it is a mono instrument. If it was a stereo instrument, with a stereo output jack, the cable you posted would work, though there are less expensive cables and signal splitter junction boxes that do the same thing just as well.
Bill, tgo