I just love posts like this!
You provide absolutely no information about what version of FTM you are using, more critically absolutely no information about your computer specs and in particular how much RAM and hard drive capacity, processor speed you have.
Further, you do not advise whether you have compacted and optimised both files before you started the merge. This is important.
Additionally you do not advise if you have media linked to the files and how much.
Nor do you have appeared to have considered merging part of the file at a time in order to not exceed your computer's apparent limitations. I am guessing here about your computer specs, as you don't bother to provide this key information.
But your problem appears to be that you are exceeding your computer's ability to handle large files.
It is always great when people in this situation then blame the program.
Perhaps you should use another program?
But if you really want to merge the files and want some assistance from this list, perhaps you could put away your frustration which does nothing to solve the problem, and instead formulate a complete query giving the list the essential information in order to advise you on how to carry out a successful merge.
Over to you