You can marry the cousins by going to "Add Partner/Ex" and then choosing "Partner with person already on tree". Following that you can add their child as you normally would.
You can also add the child under one parent, then choose the option "Change or delete", go to "Change Parents" and manually select the other person as the second parent.
(You should note that when viewing this childs family tree, the parents' shared ancestors may be visible twice. One pair will be marked as duplicate to indicate this. It is not a change in your family tree, just how familyecho displays ancestors who appear multiple times in someones tree)