There's no easy way to do it, to be honest, since there's no way to import geneaology files to branches. The best way is probably to try importing the GEDCOMs into another geneaology website that does support importing on separate branches, merge any duplicates and connect any relationships, download another GEDCOM and import it back to Family Echo as another tree.
Another harder way would be to download the GEDCOMs (or FamilyScripts) of those trees then copy/paste one entire file into the other. From there you'd have to resolve any duplicates (if they were connected in more ways than one) and connect any partner and parent-child relationships. Since the formatting is complicated for both types, though, it might take a lot of effort. You can look at these pages for info.
