Once at the competition, the Glee Club watch the other groups perform and see two boys singing a duet together. Afterwards, those boys approach the Glee club and are surprised when they find out that the New Directions hadn't allowed Kurt and Blaine to sing together.
There is a similar fic on ff.net but it ends before the competition performance and is not the one I'm thinking about.
Thanks for any help you can offer for my first search. :)