While I agree it doesn’t sound like a very good idea, there are buns that have traveled in the cargo holds of planes just fine. Look for Q8Bunny’s thread on their journey from Kuwait to Canada with her bun Chewie. We were all on the edge of our seats for that one xD Hopefully she’ll chime in here, but if not search for her and send her a PM because she’s your greatest asset in a situation like this.
The major issue with being 15 hours un-monitored is a lot can happen. Bunnies don’t eat when they’re stressed (think car rides and how most buns won’t eat during 60 minute vet trip). Bunnies that don’t eat or poop for over 12 hours can develop GI Stasis, which is fatal if not treated. It’s also such a long journey to be cramped in carrier situation, especially if you’re adopting a bonded pair (not litter mates, litter mates are not bonded and a move such as this could break any semblance they had of a bond).
If these bunnies are 15 hours away, where is the closest bunny-savvy vet? Is there one near you that you can take them to in a dire emergency?
You definitely should think about this a lot before getting them.