Did you separate the male from the females? You have to do it now and have him neutered (he can make your guinea pigs pregnant again like 5 mn after they gave birth so don't take any chance!)
It's hard to tell for the date of birth - it takes about a month as a whole and considering how fat your guinea pig is (but pregnancy shows a LOT in guinea pigs, I had one who had 5 babies once and she was enormous) I'd say less than a week.
You can house the 2 mothers together, guinea pigs aren't violent like rabbits can be and don't harm their babies. Be very careful though where you house them, baby guinea pigs are super tiny, they can go easily through holes like the one you have in your present set up.
When the babies are there, you can determine their sex almost immediatly (contrarily to rabbits, guinea pigs are easy to sex). You have to let them with their mothers about 8 weeks but not after that. You should begin to look for homes for all those babies, as guinea pigs generally have between 2 and 5 babies (3 is the more common in my experience).
