Just to confirm, do both you and your partner have a promise wristlet equipped and are in a party together? If that's the case it should be as simple as both going to Claribel (Sanctum of the Twelve, East Shroud - X17,Y18) and talking to her (again while still in a party together).

There's also a requirement that both people have completed "The Scions of the Seventh Dawn" main story quest (level 17 quest so not that far into the main story) and are level 50.

And sorry if you've already seen it, but there's a helpful step-by-step here:

http://www.finalfantasyxiv.com/etern...?lng=en&rgn=eu