Signal Integrity with Parallel Switch Control
If I understand correctly you're trying to use a DPDT switch a single wire signal from the accelerometer between 2 paths: one which goes directly to the ESC input and one which goes through a momentary NC switch before going to the input.
Given that the current aftermarket mods work through 1 switch, it's reasonable to assume the signal quality through a second switch would be adequate for the ESC to receive.  However, based on the description of the current aftermarket method for disabling the ESC, you'll likely still kill the ESC when you throw your safety switch as this will still cause a temporary disconnect from the ESC, and thus shut it down.  If you were to consider a make-before-break switch, you'll likely run into signal errors if the the accelerometer output is digital (which it likely is) due to propagation delay differences if the two branches aren't EXACTLY equal which will also likely kill your ESC.
My recommendation would be to find a mechanical safety method, such as using a push-button or toggle switch with a cover.