Will other dogs be allowed to stay with Rover sitters that aren't covered by rover insurance?

There's no reason you can't ask a sitter about this. When I get a request and have another dog scheduled to stay with me, I will always let the owners know.

If this is a major concern for you, then I suggest you search for sitters who only accept one dog at a time.

Rover sitters are independent contractors and it is always possible that they may have other customers outside of ones from the Rover site that would not be covered by Rover's insurance.