Carol you might have solved this already but it's real simple. Follow directions of previous answer. My parents married siblings too and I was concern about how to enter also, but if enter as usually would, wikitree calculates it properly. If you get stuck message me.
Also, I did place a message in the "note" section to alleviate others confusion. Additionally, on the profiles of all the children from the two marriages I mention the situation and also that the cousins are "double cousins."