Is it possible to merge 2 different Contacts archives?
I'm using OS X 10.11
Sure, copy each archive to /Users/Shared and then make two new user accounts. Import one archive in one account and export it as vcard to /Users/Shared. Repeat for #2. Go back to your Main account and import both sets of vcards in to your intended app.
At that point you can export/sync and clean up the un-needed user accounts and files from /Users/Shared folder.