The best thing to do IMHO, is prepare for the shoot beforehand. I know a guy that used a laser pointer to get the video synced. Just point it to where all of the cameras can see and a quick click and note when you did it. Then in editing just line up the flash and your good. I've also heard of come kind of clicker (makes a sharp audible click) shows up on the waveform bar really well. In both instances you want to make these marks in places that you can cut out of the final video.
At least that is what I think.
Now if you've already shot the video. It will be a lot harder. maybe pick places like the beginning and/or end of songs.
I understand that there is a multicam plugin somewhere around that might make editing easier. I don't really use speed edit so I don't know much about it.
If it's a percussion concert then you have it easy. Just find an isolated sharp drum hit on the waveform on the clip and find the same point on the other clip. It can be anywhere in the clip... beginning, end or middle.
Put a marker in the clips at these wave peaks and the markers will line up on the timeline.
Back on topic, I recently did a show, with 3 Cameras. Song and dance numbers. For the main audio we ran from their audio board to our wide shot camera (Panasonic AG-HMC40). Then all I had to do was sync the other cameras (2x Canon XHA1) to the Panasonic. To make it even easier, at the beginning of the show, one of the performers shot off a flash-paper that was caught by all the cameras.