The best thing you can do, IMO, is put their nesting boxes in a room that does not get alot of traffic, with a door that can be closed, like a bedroom. And then, as the time nears...confine the pregnant cats to that room at all times. It would be optimal if you could have 2 separate bedrooms to confine the mother cats and their litters, just in case they are more comfortable rearing their litters totally separately. But, I imagine that might not be possible in your circumstance.
Your other cat is going to be very, very curious when the kittens are born, but don't be tempted to introduce the cat to the 'new family members' (even for just a few minutes) because even though your original cat is probably very friendly and the mother cats are too, they will be VERY protective and your cat will cause them unnecessary stress. They will lash out and it can get really ugly, really quickly. I cannot stress enough that you would be better off not allowing your other cat in the room where your mother cats are raising their kittens.
Also, remember that even though you have designated nesting boxes, often they mother cats have a different idea of 'safe' and will find another place in the room that they think is more suitable (like under the bed, in dresser drawers, behind furniture). Make sure to give multiple nesting choices and block off any areas you absolutely do not want them to call home for a few months. Like under the bed, if you don't want them to nest there.
Mothers use instinct to determine the most safe area to birth and raise their kittens. Many cats prefer a nest that is covered, like a dark cave, where they can see danger coming from the entrance...but gives the feeling that they cannot be approached from all sides. You can partially cover their nesting boxes with a blanket to create an atmosphere like that.
I found that giving our mother cat access to our bedroom closet, witht he door partially open, was a good choice. I just removed everything from the floor and put down blankets and sheets for her. It was secluded and dark, yet she could see out and enter and exit easily.
you may also find that when one delivers, she may become aggressive towards the other and you may be forced to separate them. If that happens, you can get a large dog crate for the first litter so that they can share a room without fighting. Others have mentioned using them, maybe someone with experience will chime in with their ideas?
I wish you luck with your impending litters, you must have a huge heart to be taking on this responsibility. Bless you!