What you're trying to do isn't too complicated, but the components that you have aren't going to make this very easy.
The basic block diagram is simple. Bass A goes into preamp A. Bass B goes into preamp B. The unbalanced full-range output from each preamp goes into the A and B inputs on your switcher. The output of the switcher goes into your tuner. The output of the tuner goes into your amp in bridged mode. The output of your amp goes into the speakers in parallel. The AC connections all go into your power conditioner. And that's it.
If the cable runs are short (say 8' or less), then the unbalanced connectors will be fine. When the runs get long, signal loss (mostly treble) and noise may start to become a problem.
This is where your specific components start to give you a problem. Your a/b box is a floor pedal rather than rack mount. If it were a rack selector, then the unbalanced connectors aren't an issue as the cable run will probably be no more than a couple of feet. But if you need to run from the preamps to your stage position and back to the amp, you could easily be talking about a very long cable run. Also, your Crown amp only has XLR inputs, so you'll probably need at least one inline transformer to get it hooked up to the rest of the gear. The FX-1s have XLR outs that will work with the Crown amp, but this doesn't help because you really want to sum the two preamps before they get to the amp which you would probably run in mono anyway (stereo will require more speakers; biamp won't work either because you have the input selection problem again).
Life will probably be a lot easier if you can find a rack mounted a/b selector with unbalanced in and out. If you're switching mid-song, then not having a floor switching box is probably OK anyway. It would probably not be a big deal to have the guts of your StarTouch box mounted in a rack cabinet. Once you've done that, the wiring is pretty simple and cable runs are short. You'll still need one inline transformer. I guess this sort of magic box could potentially let you switch between A and B remotely without routing the signal all the way out to your stage position.
Interestingly, Alembic used to make just such a box called the IN-2 Input Module. It was a rack switcher designed to switch the stereo outputs of a Series bass. It did a lot of other stuff too that you don't need (5-pin XLR input and 3-pin XLR outputs, I believe) and would be tremendously expensive to solve this problem.
The deluxe solution would probably be to get a balanced switch box for your rack. This will definitely be a lot more bucks, but will have better performance. In this case, you will probably need the tuner on an unbalanced sidechain and, if you want to mute the amp while tuning, you'll need some sort of separate mute switch on the switching box.
The other thing you ask about - a y-cable between the two preamps - is also a possibility. To make that work right, you'll need to have a series resistor in line with the output of each preamp. I don't know what the value of the resistor would need to be to address the loading problems that you'd have from the summed outputs. This will lead to a signal loss and may have other undesirable effects on your tone. You'd probably want to do this on the unbalanced preamp outs rather than the balanced ones, otherwise you'll need two matching transformers.
Good luck
David Fung
