When a cat is calling all the normally tiny blood vessels swell with blood, so spaying does carry a slightly greater risk because of increased bleeding. In an ideal world best to wait a few days longer, however its not an ideal world and we all know the added risks involved in delaying things. Also it will be better for Millie and Ruby to be done at same time and if you wait for one to finish its odds on the other will start. If vet is happy then go ahead. Remember to ask for intradermal sutures and glue.