Leveling delay and redistributing work aren't the only means at your disposal for resolving overallocations. You can also resolve overallocations by replacing resources on a project, or by adding additional resources.
Resolving an Overallocation by Replacing Resources
In the upper pane, select the overallocated resource to show the tasks to which he is assigned.
Select the task on which you will replace the resource.
Click the Assign Resources toolbar button.
Select the resource to be replaced .
Click Replace.
Select the new resource.
Click OK.
The overallocation is resolved.
Resolving an Overallocation by Adding Additional Resources
Select the overallocated resource.
Select the tasks to which you want to add resources.
Click the Assign Resources toolbar button.
Select the additional resource(s).
Click the Assign button.
Edit the Assignment Units for each resource to 50%.
Click Close. The overallocation is resolved.
Each resource is now assigned four hours per day on each of the two tasks (50%).
In the upper pane, select the overallocated resource to show the tasks to which he is assigned. 
Select the task on which you will replace the resource. 
Click the Assign Resources toolbar button. 
Select the resource to be replaced . 
Click Replace. 
Select the new resource. 
Click OK. 
The overallocation is resolved. 
