(Project 2013) Low Priority Task with a dynamic resource allocation

Hi guys, 

dont know if this is possible but if so please let me know. 

i have to task running and they both consume 10% of each resource per day ( that 20% per day ). 

However both of the task are low priority and there consumption of the task resource should reduce if there are other task demanding resource time to prevent over allocation

Little example here ;

Resource - 'John -117' 

Day A Day BDay C Day D

Task 1  10%5%10% 0%

Task 210% 5%10% 0%

Task 3   0%90%80%100%

------------------------------------------------------------------------------

Total 20%100%100%100%

so to recap Task resource 1 and  2 should use 10% but IF there are other task they must be resourced 1st  so task 1 and 2 must dynamical adjust.

Is this possible to do ? if so can somehow provide some assistance ?  

Best wishes. 

r2690698