What MultiMerge does:

    MultiMerge allows you to set up different multiple merge configurations, which will be done all at once and saved into the directory you specify. The resulting files will be saved with their form names. For example, if you merge "Power of Attorney.frm" with "Client Data.dat" the resulting file will be saved as "Power of Attorney.wpd." You can download it from here.

First Configuration Dialog
Clicking on the "Change" button allows you to change the default directory:
Form files setting dialog
"Set" sets the directory and takes you back to the first configuration dialog. You can also edit the data file by clicking the second Edit button in the main dialog; all that does is bring up the relevant data file for editing. It does file checking, as well:

Not Found Dialog