We completed a requirement similar to yours.  The challenge as you have discovered is that while PC allows you to designate a minimum purchase quantity for individual products, there is not currently a means by which that requirement can be configured to apply to a mix of products.

 

The client who we assisted has a restaurant, and they sell perishable products that ship in a specialized container.  As such, products in "category A" have a requirement of a minimum of 4 units, and products in "category B" have a requirement of a minimum of 8 units.  Products may be mixed within their respective category in order to fulfill the minimum.

 

Our approach was to track the number of units that were selected from either category and as the customer navigates to checkout, we test to determine if the minimum quantity in each category had been adhered to.  If the test fails, we put up a message and send them back to make additional selections. (Not related to your requirement, we then also test for the presence of such products on an order when presenting shipping options and restrict the options offered to the shopper based on their ship-to location.  For local orders, we only offer ground which is equivalent to overnight.  For distant orders, we only offer overnight air freight.  This was done in order to resolve concerns over spoilage.)

 

A similar modification could be implemented for your store.  If  you wish to discuss such a project, please e-mail or telephone me.