Currently I use two online sites to post my family tree, Geni and MyHeritage.  Now on Geni other's can make updates, add people, make changes, on MyHeritage/Family Tree Builder I think its just me.  But say that on Geni users find that some of the names/dates are wrong in my tree, or add 5 new names.
What would be the best way to take that GEDCOM, and merge it with my Family Tree Builder GEDCOM, in return be able to back it up online at MyHeritage?
I am not to sure on any programs that are out there just for merging GEDCOMS.  Please leave a comment and let me know what you think.