a.1 marries b.2
c.1 marries d.2
To make these entries, I had to create a duplicate of one of the families -- can't find a way to use an existing person as a partner. How do I do this?
It is possible to attach someone that already exists in your family tree to a new relationship. At this point in time, this is possible in Family Tree Builder only, and not in an online family tree.
For exact instructions, please see the post in out FAQ: here.