You can do this, but how it is done will depend on the relationship that person has with others in your tree.
For example, when one sister marries a man from a different family, you would normally add him and perhaps build his family tree (brothers, sisters, parents, grandparents, aunts, uncles,...). If after doing this you discover that another sister from the original family marries the brother (or cousin or whatever) of the first sister's husband, he may already be in the tree. In that case, when you "Add partner/ex" for the second sister, instead of adding him, you should click the button "Partner with person already on tree". That way, they appear as both part of the first famly's tree, and a part of their own family's tree. They appear twice, but are stored once, and if both appear at the same time on the screen, one will say "duplicate".
I realize this may be hard to follow, but the key is to NOT enter someone twice, but to use the "...already on tree" option. If the person is already in the tree twice, you'll need to carefully delete one of them, then use the "...already on tree" method to connect them to someone.
