Custom Field + Discount on variant/ VM2

[lazou]

May someone could help me for this problem:

I am using VM2.
I have 2  products that need to be bought together (they are complementary). I Therefore have to use custom field and variant.
However Each product has one public price, and one wholesale price  ( it depends of with shopper group you belongs).

How can I use custom field with discount rate? All what I can see is that when using custom field I can only type the price of the variant product.
I hope I am clear enough...
In other word, I'd like to use user group to have different price on each products that need to be sold together. Is there some VM extension for this? Do I need a special script? How to proceed?
Thanks in advance for your help