(LMAO, this is the third time in an hour that I've recc'ed this author's work) - Try the Blow Me 'verse (http://rockstarpeach.livejournal.com/80400.html) and its sequel How to Date Your Brother (http://rockstarpeach.livejournal.com/90671.html).
It's really the second one you're looking for, but...In the first series, the boys start by "helping each other out" and then suddenly they're kind of in a relationship, except not. And in the second series they realize they're kind of in a relationship and try to navigate, but...well, my summary sucks. Anyway, both series are hot and funny and sweet and great.

Be Mine (http://de-nugis.livejournal.com/22324.html)
Remember how two years in a row Dean's done the "Sam, be my valentine?" joke? So Sam says yes, but Dean has no idea what he's talking about. This is the one that's famous for the phrase "gay incest love tart." I adore this story.

Slow Show (http://infatuated-ink.livejournal.com/38712.html)
J2, moves through six seasons. The boys live together and have cookies and milk together every night. Eventually they're like, "Um, are we in love?"

The Courtship of Jensen's Co-Star (http://the8threalm.livejournal.com/28511.html)
Similar to the one above. Long, slow build, but the boys are totally oblivious. Like, living together and cuddling and sleeping in the same bed oblivious.

And a Dean/Cas for your troubles: Sam Is a Genius (Everyone Is Dumb) (http://deancastiel.livejournal.com/2361927.html)
Sam POV as he watches Dean unknowingly court Cas.

I was just coming to rec The Courtship of Jensen's Co-Star. I <3 that fic so much.

The Sam is a genius fic is perfect!

These two aren't PWP, but they're NC-17 and really well written.

http://fleshflutter.livejournal.com/184372.html

http://bertee.livejournal.com/103363.html

This one might fit the bill - Dean/Cas

Why Subtlety Never Really Works With Winchesters
http://deancastiel.livejournal.com/877018.html?style=mine#cutid1

Not the requester, but do you have another link handy? I'm getting an access denied page and the title sounds really interesting.

you need to join the dean/castiel livejournal group to read it, there's no moderation so you get in right away

*facepalm* oh, I feel silly now. Thank you!

This is not 100% what you asked for, but I think it fits your criteria in spirit, if not in letter

The Girlfriend Experience by rageprufrock
http://archiveofourown.org/works/91894?view_adult=true
"While it's not like Dean hasn't had a couple of truly regrettable hit-and-runs in his sexual history, this is probably the saddest fucking thing that has ever happened to him."

Basically, Cas has a little "problem down there" that he doesn't know how to deal with and Dean has to teach him. Eventually further into the story they figure out they're in a relationship after a whole lot of Dean!denial and Castiel!confusion. Also has completely hilarious Sam that realizes what's going on and trying to push them forward.