You should export one tree as a GEDCOM file. Choose the one with the least amount of photos attached, since they have to be entered manually.
Open the other tree and then select Import GEDCOM.
Answer Yes, you want to end the current, open project.
Select Import into latest project, the two trees are (crudely) merged. Duplicates have to be removed manually, photos re-entered.

http://www.myheritageimages.com/H/storage/blogs/companyblog/...

This open source project: http://gdbi.sourceforge.net/ is said to work on a smarter match/merge function.