Are there are any rules/setups to control the allocation of work definition in Supply Planning?

Currently, 

For the Make Item, We can define the multiple work definition, and each work definition has different resource.  

In the Supply plan, based on the capacity , the work definition is allocated. Priority 1->2->3.

We are aware that it's possible to manually change work definition in the supply plan. However the requirement is as below:

Requirement:

 We would like to know if there are any rules/setups to control the allocation of work definition . Example:

Assume We have a requirement of 500 qty for ItemA. Item A has two work definitions, WD-A and WD-B.

We should be able setup in such a way that planning will allocate the 50% of the quantity to WD-A and 50% of the quantity to WD-B.