Magento – Buy one product, get one product half price

I'm trying to create a shopping cart price run whereby if you buy one of a product in one category, you get one of a product in another category at 50%. I'm trying to also maintain a 1 to 1 ratio on the discount, eg:

If I buy 1 from category A I get 1 from category B at 50%

If I have 1 item from category A and 2 items from category B, only one item from category B gets the 50% discount, the other is at full price.

If I have 2 items from category A, 2 items from category B get 50% off additional items from category B are at full price.

I've set the following on the Actions tab:

Discount Amount 50

Maximum Qty Discount is Applied To 1

Discount Qty Step (Buy X) 1

Stop Further Rules Processing No

If any of these conditions are true, category is category B

As it runs now, if I put one item from category A all of category B get the 50% discount. I'm stumped on this one…