This is a temporary thing - about 5 hours - I would not even try to get them together. It takes me anywhere from 5 days to 6 weeks to introduce a newbie into my home; and my cats are used to comings and goings! Since grandma's cat is used to being the solo princessa, why bother her?

Keep them separate and set up a second litter box for the time you are there. I mean, you can use a cheap set up. An aluminum pan from a store bought pie with some litter in it works fine! And a water bowl, and you are set. I wouldn't keep juggling them here and there. Keep the Princessa where her box is, don't expect HER to be using the temp one. Disrupt her a little as possible. And don't give your kitty run of the house either; lose her in a bedroom or, if there are 2 bathrooms close her in one.

You have enough work bombing your house; no sense in getting involved with hand grenades with claws on them!