I'll lay it down for you nice and simple:
1. If you can, position camera inside the car too look on the road and dash if you like.
2. To sync both video tracks you need either Video or Audio reference. Nice and visible like ScumLord said, man waving a flag, prefer if that was a woman, but that is my opinion.
3. Use Adobe Premiere, you can get a 30 day trial from Adobe website, and it's more than enough to do it.
4. In premiere you use "Mark Clip" and set markers in the same place in time in both video, ie. man waving flag, or car in front taking off.
5. Using "Synchronize" Option in Premiere Pro, you Sync both tracks.
6. When they are Synced, you scale them and move them around so they are taking up space on the screen the way you like it.
Basically, track preparation is the same as in case of Multi Cam in Premiere Pro. You can see tutorials on Youtube if you don't trust me