Discounts on different volumes

[maximusimo]

Hi,

scenario: I have a product that is available in 125ml & 250ml and so have a drop down menu which is named 'Volume' within combinations.

As a special offer we also offer a 'double pack' of the product which contains 2 x 250ml.

What I want is, when a customer chooses a quantity of 2 x 250ml the discount is automatically applied - any ideas?

Thanks in advance.