I would argue that pair riding down a hill on a bike would be very common for kids, regardless of whether they would be able to ride back up together or not. Firstly, it's fun and a bit crazy/wild. And the return trip back up might just be two kids walking and pushing one bike. After all, a non-e-bike would get used exactly like this by kids.

Really, giving a kid any kind of moving vehicle means new risks. Even if everything is right, crashes happen. I seriously doubt the one girl bought her e-bike with the strong consideration of it being a "two person bike". Besides, there are plenty of teens or adults who might weigh the same as those two girls together. The second seat is irrelevant.