Having spayings is the worst - for us!
Then, there's that keeping them quiet part! You'll have to not only keep them from getting hyped up, but jumping up, such as on the couch or bed. Oh yeah, and Missy from standing on her hind feet.
I do not envy you this task. What I do is just over-watch them, and keep reminding myself that it's just temporary. In a few days, we will be past this part. And, with dogs, I remind myself of the time that I spayed several cats at once, then was thankful that I was dealing with dogs, and not cats! (After that, I only spayed one cat at a time, mixing the female with as many males as needed neutering. Keeping cats from jumping up was like keeping track of a barrel full of monkeys by day 2! I finally just kept them crated that one time, and they were extremely unhappy campers!)
I think that asking the vet for something to keep them calm, just in case, is a good idea.
In the absence of Valium, you could try IV chamomile!
I'll be praying for surgeries with no complications for the girls, with fast and perfect healing for both, and no antics to run you ragged while they are healing.
Love All Life, Thank You For Posting! :)