I had two girls names (first and middle names) and two boys names (first and middle names), but if I got two boys the middle names would be swapped around so if I had a boy first we would have to wait until the second one was born to know his middle name. However as we had our daughter first then her name was 100% certain and when the second twin was a boy we knew which middle name he was having straight away.

Does that make sense?