In case you need a step-by-step explanation:
So you start with the CHILD of this endogamy (= in-marriage, cousin marriage).
He/she will get 'cousin A' and 'cousin B' as parents.
You then create cousin A as the mother of this endogamy child. Delete the father.
You work tour way up unto the common ancestors: they are then of line A only, yet.
You give the common ancestors another child to start line B. You create the descendants of this child, down to and including cousin B.
Almost there.
Select cousin A, click "add a partner", and then choose "partner with someone already on the tree" and choose cousin B and "add as partner".
Select the child of cousin A, click "add father" and confirm that he is cousin B.