If I understand right, you've got for example.. John and James are brothers. Anna and Sally are sisters, in a different family. John and Anna marry, and then Sally and James marry. If this is the case, I would take whatever family you're starting from, say the guys..
You start with John, and add his brother James. Then, you add John's wife, Anna. Then, clicking on her name, you add her sister, Sally. The click the sister's name and click 'add partner with person on the tree', and find James' name. It might give you some weird duplicate names but it should work.