Are the 'main storyline quests' solo only?

I was playing with my brother last night and twice we did quests that wouldn't allow us to play together, it forced solo play. They may have been involving the Prophet, which is the main storyline.

We were on the same leg of the same quest.

Is that how it works, or was something busted?

If it was supposed to be that way, those quests should be marked (solo).

Yes, that is the case and I agree it should be made more clear.

Ran in to this with the guild quests as well, tried to do the second Mages guild quest with a guildmate and found ourselves in different instances no matter what we did.