No. Mia and Sebastian are no longer together in the movie after they decide to officially part ways. We see this in the movie when Mia and Sebastian, after struggling all along with their career and facing rejections, finally decide to part ways but with a promise that they’d love each other, no matter what the circumstances, even if that
It’s fine they don’t end up together; in a movie that’s in love with that past, that’s the freshest thing La La Land has to offer. In every song-and-dance number, La La LandI'm with you, the ending is what makes the movie, and the ending is what makes all of the best movies in my opinion. I'll use A Star Is Born as an example, good movie for the most part, absolutely terrible ending. What I love most about La La Land's ending, is that they do end up together in the alternate timeline.
These things both manage to propel each person's respective dream, but the reactions from the other are why they can't end up together. So they each achieve their dream, and the core of the movie is that pursuing dreams is a foolish thing you may have to give up a lot for (a theme which Audition wears on its sleeve). It's a simple one, done well. .