This process is known as creating a ‘Cycle’. A Cycle occurs when the same person appears more than once in your tree (for example, when two sisters marry two brothers).
You must be viewing the tree with standard view or flipped nodes turned on. Use the ‘Move this Person’ button found in the ‘more’ link of a node on your tree. This will add the node to the Sidebar, found on the side of your Tree view. Then drag the node onto the second person to whom he or she is related and select the appropriate relationship.
**Note: If the 'Move this person' button is not available from the drop down 'more' menu, you will first need to click on their 'tree' link to center the Tree to their perspective**
You can also create a Cycle by navigating to one of the nodes, holding CTRL (Command for Mac users) and left clicking the node, then drag the node onto the second person to whom he or she is related. Then select how the two nodes are related.
Once the Cycle has been created a small green circle will appear on the lower left-hand corner of their node, with ‘x2’ in it. To view the other relationship, you may click on the circle and it will redirect you to the other relationship.