Help/Thoughts on builing a fuzz/phaser

The only parts of the boards that would overlap would be the power supply, switching, and in/outputs.

The power supply would just need modification to both circuits so that the 9V was split equally, and then reduced down to the proper level needed at the first stage of the circuit. That will require analyzing the amount of voltage drop in the unmodified circuit, where the 9V first hits the circuit. We're probably just talking about one or two resistors to be changed (for each of the boards).

The switching and in/outputs can be very simple if you run them in series. Basically each circuit would have its own footswitch, and the out from one would connect straight to the in of the other. It only becomes complicated if you want like one bypass switch and one A/B switch, or parallel outputs, etc. Even that is highly do-able, just not as dead simple as running them in series.

IOW unless you have some fancy plans, all you're doing is putting two completely independent circuits, each with their own switches etc., in one box. In that context the only thing you would "bridge" is the power supply. The audio connection between the first and second circuits is hidden inside the box, but aside from that it's built the same as if you were building them separately.