This following is NOT my own answer, but rather one that Family Echo gave on a question similar to this one:
I'm afraid it is not currently possible to merge two or more trees together in Family Echo. It is a feature that a lot of people have asked for and will hopefully be added in future. You can save multiple trees in one account though, and access them using the 'My Account' link in the top right of the page.
Hopefully this feature will be added in the future.