How to add flat rate discount for multi product purchase?

[gowri]

Hi
I need to give an offer for multiple product purchase
When the customer purchase multiple product from specified category
We will offer a fixed amount for the purchase.

Example
Product - 1 price of the product is $20
Product – 2 price of the product is $25
Product – 3 price of the product is $30
Product – 4 price of the product is $27

Buy any 2 from this four at $30…

Customer can purchase any two products from the four at $30 only….

Please help me how to do that one

Thanks

[tomerg3]

Check out my post http://www.prestashop.com/forums/viewthread/75558/