If you're using an A/B switch, you don't need to turn the volume down of the bass that you are not using. If you have it set to loop A (where bass A is plugged in) you won't get any signal from loop B (where bass B is plugged in) going to the amp.
Essentially you can select via the foot switch:
Loop A (with bass A plugged into it) -> other effects (if any) -> amp
OR
Loop B (with bass B plugged into it) -> other effects (if any) -> amp
If you were using an A/B/Y pedal, the Y function would turn both loop A and B on at the same time and you would need to turn down the volume on the bass that you are not using.
If you're looking for an A/B pedal, or an A/B/Y pedal,
loop-master is a great place to buy from. There's tons of options for A/B and other types of bypass and switching/routing pedals on there and they have fairly decent prices. Definitely not as cheap as making your own, but pretty fair. There's a bit of a wait for them to build the pedal (they're generally made in batches and sometimes they don't have any of a particular pedal in stock), but they work perfectly fine if you need a really cheap solution.
However, the downside with a simple A/B pedal occurs when you are using two basses where the output volume of each bass is noticeably different from the other. Another downside occurs when the impedance of two basses is different, such as in an active bass and a passive bass.
For instance, lets say that the output of bass A is moderate for a passive bass, but bass B is a high output active bass, and you have them both plugged into a passive A/B pedal. In this case, playing bass A in loop A of the pedal, setting it down and then playing bass B through loop B of the pedal will give you two different output volumes, and your pedals will get two different impedances. (honestly, most of how impedance works goes right over my head. If you really want to know how it works, I suggest doing a search on here, google, or just waiting for the resident electrical engineer to pop in and drop some knowledge
)
The output volume can generally be equalized by setting your bass volumes on each bass so that they are much closer together, which isn't so hard.
However, the impedance issue is one that a simple passive A/B box can't overcome. For that you need an impedance-matching box that will correct the impedance of the signal coming from both basses to be exactly the same (There are few products out there that can do this) but suffice it to say that unless you own a picky effect like the Z.Vex Woolly Mammoth which likes to see a passive bass' impedance (it still works with active bass, but doesn't sound the same), you really don't need to worry about impedance issues.
Another thought is that if you just run your bass direct to the amp, and your amp has two 1/4" inputs like on most Markbass amps, you could just plug one bass into each input, adjust the input gain as needed, and turn the volume up/down depending on the bass you need to play.
