How to apply discount for 2 of the same item?

I’d like to apply a discount to an item if 2 have been added to the cart.. example

item 1 is $20

a customer add item #1 two times to the cart, now the price would be $40, but i want to price to be $30 instead, giving a discount of $30.

how??

Thanks already

http://wordpress.org/extend/plugins/wpmarketplace/