(click on above image to view a readable diagram of the wiring)

Basically what is outlined above requires the modifying of the MAP sensor inside the MegaSquirt to be used by an external source. First trim solder leg #1 (you can use any unused pin on the DB-37 connector) on the MS DB-37 connector (it needs to be independent from the ground circuit on the MS board), then wire a jumper from pin #1 (Vout) on the MAP sensor to the cut leg #1 of the Db-37 connector. Now you are able to share the MAP.
From there the wiring is just sharing of the sensors between units. Different variations can easily be adapted such as 2 or 4cyl direct ignition (use this wasted spark diagram). You can even use the injector output of the SMT for a turbo boost solenoid (need to change the software NOX setting to "on" and use a high speed relay between the SMT and the solenoid, output will pulse at 38Hz, map settings will change the pulse duration time). This would allow you control of the boost curve if you have a traction problem
The "map A" and "map B" options are changed by the SMT #4 wire. Basically the SMT is able to have two completely separate tune maps that are switchable on the fly. This has more possibilities that can be imagined but here are a couple....

Gear Change REV Limiter: use the map B option to retard the IGN timing at the desired RPM. A simple micro switch attached to the clutch pedal would allow this option.

NOS IGN timing and progressive NOS control: use the map B option to retard the IGN timing when NOS is activated and use the injector output to control the NOS solenoid, this would be fully mapable so Nitrous can be activated at very low RPM's and mapped out to have max power with minimal wheel spin. A simple micro switch attached to the NOS activation switch would allow this option.