CBSubs Promotions certainly lets you apply discounts if you have X plan active. For example lets say you have the following setup.
Plan A
Plan B
You could have a promotion that if purchasing Plan A and Plan B is active apply 50% discount. You'd have a second promotion that does the reverse of if purchasing Plan B and Plan A is active apply 50% discount.
We provide backend and frontend access to CBSubs on our demo site below if you want to explore CBSubs Promotions.
demo.cbdemosites.com/
After using the "Login As" for Administrator and logging in click "Community Builder" on the menu bar at the top of the page, mouse over Plugin Management, then CB Paid Subscriptions and click Promotions. From there Promotions Settings lets you create new promotions.