Plugin, Edit the selecting product

[massyc]

Hi, i need a plugin or a method, that change the classic selecting product of virtuemart, let me explain:
When a customer or user select a product automatically the system add a second product, but not the same product, one choose by me.
How can i do? i need a plugin like: VirtueMart Purchasing Rules but it must work with virtuemart2.
Someone can help me? i really need a help.