How to replace product quantity attribute if there's a multiple instance of that Product?

The scenario is this:
By configuration, Product A has a component Product B.
Product A has also the attribute Quantity which refers to the quantity of Product B.

Ex:  New Install Order

    Product          Prouct Qty   Action Code

        Product A          1             add

           Product B       5             add

     

    Attribute: Qty = 5

Problem:

        Modify Order to add another instance of Product B

                 Product                 Product Qty    Action Code

                 Product A                    1                  -

                      Product B                5                 -

                      Product B               10             add

 

           Attribute Qty  = 15   ; this is what happens now having the constraint  "On select of Product B requires Attrib:Qty =  Qty of Product B"

           Requirement: Attribute Qty should have the value of the latest product instance;  Qty should be equal to  10 and disregard previous instance of the product.