Resource with multiple assigned amount at same task


I have the following situation: one resource is assigned to 2 different tasks, and these tasks have different start and end date/time. In a given moment both tasks become parallel, and resource become 200% allocated. If I reduce the amount of time for the resource in each task to 50%, the total amount reduces to 100% when in parallel, but also reduces to 50% when not parallel.

Question is: is there a way to have the resource 100% allocated when one task only, then 50% during multiple-task, then 100% back when one task only again, so I can have my resource all the time 100% allocated?