
Dogs are not supposed to share benches. If they were put on bench together it would suggest they were not happy being benched and it would be most unwise to leave them for more than a short time.
As to how long you leave your dogs, that's personal, as many will of course be perfectly happy to rest on their benches and of course the owen5r may be busy showing another dog, waiting in the long queues for the ladies trying to get from a - B through the press of people at the show. This is especially likely if they are showing more than one breed that may be benched far apart.
Certainly had this happened in my breed someone would have untangled the dogs and then reported the issue to the owners on their return, so hopefully ensuring they were not left at risk again.
We do generally watch out for each others dogs when the owner is away from their bench, and I'd certainly ask neighbouring exhibitors to watch out for them and let me know if there was any issue while I was away (more causing nuisance, perhaps by barking).
If a dog has been trained to bench happily there is unlikely to be an issue, the worst is if the benching chain gets caught in the join in the benching that you sometimes get.