Intervals has a one to one relationship between owner and assignee. We believe this relationship helps keep things from falling in-between the cracks. Although this one to relationship exists, there are a few ways to handle the scenario:
A task can be assigned to different people in succession - we often do this where John has the first part, then over to Jaime, then to Steve to finalize
The task can be broken into multiple tasks
Multiple people can work on the same task - since Intervals features project level permissions for project teams everyone can see all of the tasks for the team they are a part of. It is not uncommon for us to say "hey, lets all work on task #345 this afternoon for an hour".