I can tell you from personal experience that this is very difficult to achieve. The speed of a film projector isn't a solid 24 fps and you'll end up going out of sync even if you manage to hit the play button on the secondary source fast enough.

PS I am assuming that you are thinking of running the film print through a projector and the sound through a cd player or mp3 player... This is what I had to do when the answer print for a film festival I was projecting had sound that was way out of sync, and we had to burn the sound files to a cd, and sit next to a stereo with our finger on the pause button, waiting for the smpte countdown to hit 2. We would sometimes be off by a few milliseconds and that would mess the whole sync and no way to turn off the projector and rewind.