With phpMyAdmin. go to the profiles table and find the member you want to convert to couple.
Edit that entry. make a note of the ID number. You will need it in the next step.
Now empty the ID field.
add a (2) to the end of the nickname and email address. So you have nickname(2) and my@email.com(2)
Now at the bottom of that phpmyadmin entry where it says save, select insert as new row and click go.
Now you need to find this new entry.
Edit it, and in the couple field enter the id number of the original member entry.
Then go back to the original entry, edit it and change the couple field to the ID of the new entry.
This ties the 2 accounts together.
Now edit the new entry again and change other info such as gender, dob, desc, ect to match the second member.
Now empty the cache.