How do I specify tasks to achieve the following:

Consider the following (simplified) set up:

• there is a task A that starts on Jan, 1st and consist of a work amount of 6 months, if a resource is working on it 100%.  So it will be finished end of June
• there is a task B that starts on March 1st and goes until end of March and the resource is allocated 20% to it

Now in this situation the resource histogram shows that in March the resource is used to 120%.  That is not what I want.  I want that the resource utilization stays at 100%.  That it detects that in March the resource can only work with 80% on task A.  And as of course the work amount of task A stays the same the work of 4 weeks at 20 % (= 0.8 weeks at 100% = 4 days at 100%) is added at the end so that the end date of task A automatically shifts out by 4 days.

I tried to achieve this by setting in the Advanced tab of the tasks the type to 'fixed work' and giving task B higher priority than task A in the General tab (hoping that then task B would be serviced and task A shifted out).  But that did not happen.  The end date of task A stayed unchanged and in March the resource stayed overloaded.  How do I have to set up the tasks to achieve an automated distribution of the work amount?