The text for Dream says
If the target is asleep, the messenger appears in the target's dreams and can converse with the target as long as it remains asleep, through the Duration of the spell. The messenger can also shape The Environment of the dream, creating landscapes, Objects, and other images.
So what happens if someone else is already in the target's dream when the spell is cast? Would the two messengers be able to converse with the target simultaneously? Would they even be aware of each other? Which one would get control of the environment?
The situation may sound absurd, but one could imagine an important person who wants his dreams to stay private might employ someone to cast Dream on him each night if it would prevent other people from doing so.