You need as much info about the one having kittens as possible including an emergency number if there are probs.
Need to know what signs that she may have just before birthing starts.
You need to know about making places for her to nest in and have the kittens....how long delivery takes, how you know when its over, what happens if the kittens dont feed or one is born with a problem and she rejects it. How you feed any rejected kitten and what with, how long etc etc
The other one you need to know age, weight, what her weight should be, fleaing and worming for both.........may not be possible for the other one at moment, spaying , vaccinations.
Any medical probs with either.........I hope they are both healthy
all the things you feel you are not sure about and then we can possible help you withb things that you then think of. Tell the vet you are new to cats so need as much info as possible.
Hope the vet trip goes well