GREG LUMLEY, PETAPIXEL:
"Shooting two or more cameras generally means you need to ensure the cameras are all synchronized to the same clock time. Unfortunately, my Canon 5D Mark III drifts horrendously when it comes to keeping good time — perhaps even 20 seconds in a week. I found myself continuously having to set each of my cameras before each wedding shoot to ensure images are timestamped in the right order.
"After giving it a think, I came up with this handy way to sync my two DSLR cameras using Lightroom and a 'Timestamp' trick."
Read the tutorial, from here.