Is there an easy way of setting up a discount which is applied when a customer buys 2 products within a category.

For example Product A is £15
Product B is £20

Customer buys 2 of Product A it's £30

If they buy one of Product A and one of Product B they receive a discount, for example 10% and the total would be £31.50.

I need to apply this to one category only though, any ideas?