I don't know of an app that does this, but if you added a countdown at the beginning of one, you could know when to hit play on the second video, and then they should be pretty close to synced. Or you could add timecode to the videos, if you have FCP, and then you'd be able to line them up exactly. Another option is to combine both videos into one video. Just scale them to fit your aspect ratio and sync them on the timeline. This would accomplish perfect sync and display both at the same time.