I have Application Gateway with url based routing, /app1/ points to app1 app service, and /app2/ points to app2 app service.
I have domain test.com pointing to Application Gateway. So test.com/app1 routes to app1, and test.com/app2 to app2.
Next step is to configure ssl settings for both app services (I do not use ssl offloading here). Added test.com custom domain to app1, configured ssl bindings, adding a custom domain to app2 fails with a message:
Hostname is already being used in the following App: app1. Please remove the Hostname from app1, and try again