how join 2 activities in one

I've followed this suggestion to combine 2 runs.  Downloaded + combined.  However, on Upload of new combined FIT file, I get an error from Garmin saying the file is in error.

What does the error message say, specifically?

The problem with the procedure provided by RobCuadro is that by default the resulting fit file will have the same id as the first of the unmerged activites, so Garmin Connect will refuse to upload it, saying that this activity already exists. So you have to delete the unmerged activities from Garmin Connect before trying to upload the merged activity. 

The message says "We don't support the file type you tried to import."  It does not mention any other detail.  I don't want to delete the unmerged activities without first confirming that the merge file is correct.  Is there a way to change the ID of the new (merged) file which I'm trying to upload?

If you can zip the 2 files you want to merge and attach it to this thread, we can try to merge them for you.

OK, cool, thanks.  Here you go.  Appreciate.[solved]

This will work - [solved]

Merged using Fit File Repair Tool

It worked!  TYSM