Question:  How do I setup "Buy Item A, get B at 50% off"

Answer:
For product A add a role "effective on add to cart", this will add a user to a role when they add product A to the cart.  Here is a help article on adding the user to a role:
https://helpdesk.smith-consulting.com/kb/a1563/product-roles.aspx

Next, setup product B with a "role discount" of 50%.  Here is a help article on role discounts.
https://helpdesk.smith-consulting.com/kb/a1536/discounts.aspx