How to set up a Promo to meet described requirements

We are developing a promotion where a customer buys product A and product B and gets a new price on product B.  For each A and B they buy they get the new price on item B.  We have set this up several ways but come up with a problem in each scenario.  The problem is that as soon as the customer buys 1 A and 1 B they will get the new price on as many other Bs they buy.  So if they buy 1 A and 5 Bs they get the new price on all 5 Bs.  What we need is that they get the new price on the