this is driving me crazy I'm not kidding, this is about to drive me bat-shit crazy.
It's a post 513 fic. Brian had been making trips to NY to see Justin while looking for office space for Kinnetik (Justin doesn't know about the search for office space). Justin fucks some guy named Bill, and Bill decides he wants Justin to be his twu wuv. So, when Justin is visiting the Pitts, Bill shows up to surprise him and assumes Brian is the "other man". Brian leaves and eventually opens the office in NY, and Justin never knows until Michael lets it slip about all the side trips to NY. Of course it ends happily ever after.
Does this sound familiar to anyone? If nothing else, just to know I'm not making shit up. *crosses fingers*
ETA: I took everyone's advice and posted on getithere and got a response. It's Reunification by comedownstairs