Our twins' first friends when they were only about eighteen months old and in day care, were another set of boy / girl twins who are only a few months older. The four kids have continued to be friends ever since that, despite going to different schools. Our daughters go to dance class together, and our sons will be playing baseball on the same team with their dads as co-coaches.
We have had double play dates, whole family visits to each other's homes, and split play dates - the boys go to one house and the girls to the over. Then, the next time we switch.
This time it was our turn to host the boys. They almost wore me out last time we hosted them!
But now it seems that due to a scheduling conflict, only the girls will be able to get together. Unfortunately, my daughter just had a play date with a school friend, and my son has been waiting to have one of his own, but it has been difficult pinning one down from the kids' parents. Of course, this is difficult for a six year old to understand, especially when his friends have been inviting him.
So my wife spoke to the twins' mom and suggested that it wouldn't be fair to our son if our daughter had another play date by herself. Our friend totally understood (because she has twins of her own) and so we are going to try to coordinate a different get together for the families, even if it is not a full play date. This is definitely a twins issue!