User blog comment:JAlbor/Dawnguard: What Everyone Should Know/@comment-94.196.38.189-20120629163224/@comment-3524429-20120630012003

You can only pick one side per profile. You technically join the Dawnguard first, but you have to return Serana (Harkon's daughter) to Castle Volkihar either way. When you arrive at the castle, Harkon will offer to make you a vampire. If you accept, you join the Volkihar vapmires; if you decline, Harkon banishes you form the castle and you remain with the Dawnguard.