I've yet to do this, and itch works in peculiar ways so I'm not entirely sure if there's something else that needs attention, but: If you click on your profile and go to 'Sales & Bundles' there's an option to set 'Sale Conditions' where you can check 'Must Own XY Game' for the sale to be accessible to them. I would say you'd have to create a sale only for those who bought the other game, and then create another one for the bundle, so that it's be easier for some to buy them both at a discount.