So sorry to hear about Darcy, and bless you for taking on adult cats in need.
I foster, and it can take a while for cats to settle in, eg I had a brother and sister in recently. He took it all in his stride, she spent 2 weeks hiding under or on top of the bookcase. I tend to leave them to it and let them come out in their own time. One of the probs was the orginal owners got a puppy who became a huge adult dog, and was a bit loopy to say the least. My dogs are fairly elderly now, and she did get used to them in time.
An older couple came to see him to see if they would like to offer him a home. In the time it took me to find him (top of the stairs) his sister had got on the chap`s knee and started purring. Talk about selling herself, they fell for her hook, line, sinker.
One hour later and one very happy little lady patrolling around her new home.
I hope your 2 new babies soon chill and come out of themselves. Sounds like you are doing everything right.