Here (http://avengers-search.livejournal.com/528716.html) and here (http://avengers-search.livejournal.com/213812.html) are similar searches.
You can also have a look at a theme: hidden identity (http://avengers-search.livejournal.com/tag/theme%3A%20hidden%20identity) tag here or Secret Identity (http://archiveofourown.org/tags/Secret%20Identity/works) and Identity Porn (http://archiveofourown.org/tags/Identity%20Porn/works) tags on AO3.

Or do you search only for fics where Steve and Tony are dating while one of them is keeping his other identity a secret from the other? Like, no to fanfics where they're just friends/acquaintances/enemies and only get together after the secret reveal? In this case:

MCU:

Toy Soldiers (http://archiveofourown.org/works/3532880) by copperbadge
Every Villain Is a Hero (in his own mind) (http://archiveofourown.org/works/2339012/chapters/5155331) by AvocadoLove

MCU/comics:

Alter. Ego. (http://archiveofourown.org/works/485818) by arysteia
Masquerade (http://archiveofourown.org/works/552683/chapters/984847) by AnonEhouse

Marvel 616:

Luminosity (http://archiveofourown.org/works/1045470) by captainshellhead, vibraniumstark
Hypotenuse (http://archiveofourown.org/works/227890) by derryderrydown
Secrets of a Successful Marriage (http://archiveofourown.org/works/247356) by valtyr

Marvel Adventures: Iron Man/Marvel Adventures: Avengers:

Some Assembly Required (http://archiveofourown.org/works/89825) by valtyr