How do I allow two resources to do the same job and have the duration change based on the fact that I now have two resources instead of one? Can you explain effort-driven?

When you assign or remove people from a task, Project lengthens or shortens the duration of the task based on the number of resources assigned to it, but it does not change the total work for the task. This is called effort-driven scheduling and is the default Microsoft Project uses when you assign resources to tasks.

As resources are added to a task, the total work on the task stays the same. The amount of work distributed to the resources assigned to the task, however, changes.

Effort-driven scheduling only takes effect when resources are added to or removed from a task. Effort-driven calculation rules are not applied when you change work, duration, and unit values for resources already assigned to a task.

When working with effort-driven scheduling, keep the following in mind:

The effort-driven calculations apply only after the first resource is assigned to the task. After a resource is assigned, the work value doesn't change as new resources are assigned to or removed from the same task.
The effort-driven calculations aren't 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 doesn't change as new resources are assigned to or removed from the same task.
If the assigned task type is Fixed Units, assigning additional resources shortens the duration of the task. Learn more about task types.
If the assigned task type is Fixed Duration, assigning additional resources decreases the individual unit values for resources.
If the assigned task type is Fixed Work, assigning additional resources shortens the duration of the task.
Summary tasks and inserted projects cannot be set to effort-driven.