There are a few steps to do what you want.

First export your current bookmarks to an HTML file as shown in the exporting bookmarks to an HTML file article. Next restore the bookmarks from the past date, as you mention this will wipe out your current bookmarks. Finally import the bookmarks from the HTML file. Importing from an HTML file adds the bookmarks to those that already exist.

You may end up with duplicate bookmarks by doing this, in which case the CheckPlaces add-on can be used to remove duplicates.

You can also select the bookmarks and copy the selected bookmarks to the clipboard before restoring the second JSON backup and paste them back afterward. I would recommend to make an HTML backup before doing that.

See also http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox