If you have had them separated from the flock, it would be best to add both mothers back int the flock at the same time. Much better than adding a single bird to a flock.
If you have had them separated from the flock, the chicks will be strangers to the flock, and probably should not be added until they are bigger, however, with that many of each size, and only a week apart, I would combine them not expecting a great deal of trouble, maybe a little scuffling. I would supervise, but with enough space, it should go pretty good.
I keep my broodys with the flock. They introduce the chciks, and keeps the layers in line. It works much better I think. But I myself, and challenging things this week, I have a hen that hatched out 3 weeks ago, and two days before she hatched, another went broody, those are going to hatch any day. So we will see if I think this is a good idea in a week.
Mrs K