DL;
Depends. On an Asia flight I recently DHed on, I sat next to a FO who was going along for the entire trip. The LCA stated that the FO was not qualified until he gave him his line check and he was not going to do that until the leg home. Ergo, he could not sit up front with someone besides another LCA. SO this FO got to DH all over the world.

I find it weird, but this guy was doing it. No kidding.