I have an app A and an app B. App A has a yearly purchase with access to just A and a combo that gives access to both A and B. Similarly, app B has a yearly purchase with access to just B and a combo that also gives access to both A and B. As far as I am aware, this is the only "subscription across apps" method that Apple support (you have to have individual combo purchases in each app and roll out an account system to allow people who purchase the combo in A to get access in app B).
The problem is when a user who already has the yearly purchase for A wants to upgrade to the combo in app B. It's not an option for them to cancel the yearly purchase and then purchase the combo because then they would still need to pay for both subscriptions until the yearly one expires.
I see only one viable solution:
When the app detects a user wanting to purchase the B subscription when they already have the A subscription, the app can tell the user to go to app A to upgrade to the combo instead.
Am I missing some alternative solution that provides a better UX?