You can do this, but it's kind of tricky, and unfortunately the merged tree with lose any photos you may have, which will need to be reimported.
First, ensure there are no duplicate people between the trees, except one of you should appear as the spouse of the other on *one* of the trees only. This person will be used as the connector.
Now in each tree in a browser lke Firefox/Explorer (not Safari/Chrome), click on the connector person, and see the 5-character ID that appears in the browser URL, after the colon (:) - copy both of these to a text document.
Now download both families as FamilyScript. Then open each FamilyScript in a text editor and search-and-replace the 5 character ID that you found for the connector in that tree with something new like JOIN1. (You should check this ID is not already used in the FamilyScript, but you'd be really unlucky if it was!)
You'll probably also need to search-and-replace the ID START in one of the trees with some other ID, since it was probably used in both trees for a different person (unless it so happens that the connector person was the founder of one of the trees).
Finally you should be able to combine the two pieces of FamilyScript together into one file, then import it into a new tree in Family Echo, and you should have a combined family.
BTW I do understand it's not necessarily worth all this bother for you, but hopefully someone might find it useful!