If you initially assign 2 resources to a task with a duration of 24 hours, Project schedules each resource to work 24 hours, for a total of 48 hours of work (Work, not Duration!).
The resources each work 24 hours, on a task with a duration of 24 hours.
Start over and create the same task, with a duration of 24 hours, and assign only one resource at 100%. Work for the task is 24 hours.
Then later, add a second resource at 100%. Effort-driven scheduling will cause Project to schedule each resource to work 12 hours on the task.
Total work for the task remains 24 hours and the task's duration is reduced to 12 hours.
Even though the effort-driven setting is turned on by default, it doesn't make a difference in your scheduling until you add resources to or remove resources from a task.
Effort-driven scheduling behaves differently depending on when you assign resources:
- Effort-driven calculations are applied only after the first time one or more resources is assigned to the task. After a resource is assigned, the Work value doesn't change as new resources are added to or removed from the same task.
- Effort-driven calculations are not applied to multiple resources that are assigned at the same time and that are the first assignments on a task. After this initial assignment of multiple resources, however, the Work value does not change as new resources are assigned to or removed from the same task.