I don't think you can just parallel the two as one could be fighting the other and possibly fry something.
If each signal wire passed through a diode before being combined, then there would be no chance of backfeeding either one. The diode would result in a slightly lower signal voltage though, and might be noticeable.
Another option would be to have a switch somewhere to switch between throttle and PAS. Ideally this would be built into the throttle so as soon as you moved it slightly, it switched over.
"One test is worth a thousand opinions"