Is there a way to merge two family trees that are linked by a common person, like a cousin?

Answer 1

Doing various things to GEDCOM files (like merging and splitting) is pretty common and there are a number of utilities to do that sort of thing. There isn't anything built into Family Echo, but if you search for "merge GEDCOM files" you should get some options. Note that GEDCOM files do not contain photos, so any work with a GEDCOM file will not keep photos.