Ok, so I kicked both cameras down to 1080p instead of 4K, figuring it would reduce the heat build-up. And I managed to get the expected 4+ hours out of camera #1. Great!
However, camera #2 is still having problems – it would shut down as soon as I hit record. I tried doing a factory reset, I tried swapping SD cards, but the problem appears to be that it just doesn’t like the sidecar battery. It doesn’t matter which sidecar battery I use – as soon as I turn it on, it turns off. Take off the sidecar battery, and it’s quite happy recording for 100 minutes.
I think for this weekend I’m going to try using the sidecar battery on camera #1, but just going bareback on #2. I’ll probably put #2 on the bow facing back, because that view is usually only important in the first part of the race. When I get home, I’ll see if GoPro will admit there is a problem with their camera, although since it only happens with a third party battery I don’t have high hopes.