
I think I'd be using the alternative, UNLESS you can talk to your usual sitter and STRESS that your pup is not to be with the other puppy - at all. You'd have to trust her that she listens, and acts!! If there's no need to use stairs (to go outside?) with the alternative, then provided again you say he's not allowed on stairs, and tell her why! this might be best.... people do tend to say yes, yes, and carry on doing exactly what they want to be doing. Obviously if your puppy is injured, you'd have recourse (is she insured?) but it's a bit late once your puppy IS injured!! And at the end of the day, there's no NEED for these two puppies to be together - this sitter should know that!
We didn't take holidays, apart from once, when we had numbers of hounds - too expensive to kennel them, apart from not wanting to leave them with ANYBODY. The one time we went to Europe for 9 days and a neighbour sat with them. Everytime the mobile rang, which it did every time we moved into another area, I panicked. They were fine but whether I had a good break, is debatable.