off we go, into the wild blue yonder

There was nothing organic about their bond, the Not-Donna noted with a huff, averting her eyes from the Not-Doctor and suddenly finding the window to the vast darkness that engulfed them the most interesting thing in the universe, those were merely echoes of the bond the Doctor (the real one) and Donna shared - however, knowing that did not stop her heart from racing when he looked at her.

-
or
the one where the Doctor and Donna are stuck in the ship in WBY for a little longer than just a day or so, and end up finding out things about themselves through the Not-Things.

[ALSO POSTED ON AO3]