Yes, when you edit the videos you can put them back together. And yes also on all their cameras splitting up large files. In 1440p on the Black GoPro 3 the files are only about 15 minutes each.
The problem with carrying multiple batteries is that you need to reset your date/time each time you remove the battery. That can be a hassle on the water. Also, I believe that the file numbers reset each time you remove the battery (you start over at GoPro001).
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