A Switching Box. I decided to keep it simple(!) It's the black box bottom middle.
Two effects loops, I'm using A for distortion pedals and B for modulation effects. HPF, Tuner, Compressor and EQ are before the box.
Buttons are, left to right: A/B --- BYPASS --- AND/OR.
The 'AND' function is series not a Y or parallel. Everything is true bypass, except that when B only is selected the input of A is still connected to the input - this doesn't cause any detectable loss of signal. I couldn't find a way to avoid this using no more than 2 switch poles. This does mean effect in loops A will bleed through to the output if you run the box backwards, so it is unidirectional!
The A and B LEDs light dimly to show the configuration when the bypass is active so you can tell what will happen when you turn bypass off, done by putting an extra 4K7 resistor in series. Some crude diode logic allows all the LED switching to be done with one pole of each switch.
The A/B jack sockets are bypassed so if you don't have anything plugged in there's still signal continuity BUT plugging in either send or return without the other will break continuity.
Power connector not shown on circuit diagram, but fairly obvious where to connect it. Box will work without power, just no lights!
Working this out was a bit like doing suduko and cost me a night's sleep!