Interesting idea. I suppose you could wire up 2 PCMs, then have some kind of switch to control which PCM is active...
The first problem I could see though, would be that if you switched off one PCM, drove the car on a 2nd PCM, then went back to the first PCM... then you'd probably need to do a CASE learn afterward. Perhaps there would be a way around that (say switch off the outputs but leave the inputs active).
Perhaps a different way to accomplish your goals would be a happy knob or switch and tuning to make the knob/switch very effective? Then you could add/remove say 5 degrees of timing on the fly. You could wire the same switch to turn performance shift on/off... might give you some kind of "track mode".
