Can a discount be applied to item A when another item B also is on the order

Summary:

Need to be able to apply a discount to an item when a different item is also ordered. For example item A can be ordered on it's own and costs $100, however if item A is ordered and item B is also ordered then item A should be discounted to $50. Can this adjustment be made automatically with a discount rule?

Content (required):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):