I want to merge the files, but do not want FH to do any merging of individuals or families, even if they may be identical people.
I strongly recommend that you take a backup before making this kind of major change to your file - in case you make a mistake and want to rollback.
Having done the backup...
Open the first file as normal, if not already open. Then click on File > Merge/Compare and select the file you want to merge in. Family Historian will spend a while looking for record matches between the 2 files, before the Merge/Compare dialog opens showing the records from both files and whatever matches F.H. has found (possibly none). The Merge/Compare dialog has multiple tabs - one for each record type. You need to do the following for each record type (i.e. each tab).
Click on the first row of the Info Pane (area on the left). Scroll to the bottom of the list, and keeping the Shift key pressed, click on the last row. This should select all the rows in the Info Pane. Click on the Unmatch button. That will unmatch any records that F.H. has matched (within the current tab).
Repeat this, as I say, for each record type. Having done that all matched records will have been unmatched. Now click on 'Merge'. The 2 files will be merged but no records will have been merged.
You only actually mentioned Individual and Family records. Obviously if you are open to F.H merging some records types other than Individual or Family (e.g. Source records) you don't know have to unmatch those.
Simon Orde List Administrator & Family Historian designer