The parsing schema for an ApEx application
For an ApEx application, you can define one parsing schema per application. For smaller applications, this usually will be sufficient. For larger applications however, there might be the need to address multiple database schema's from one application. Because there will always be one parsing schema per application, you would have to grant the appropriate privileges to a schema in order to be able to use DML on all objects needed. This can be a daunting task, especially when you have many objects in various schema's.
Instead of using only one application, you can use multiple applications. This may sound rediculous, but using more than one application, you can use a different parsing schema for each of these applications. The real trick is to get these applications to work as one. And this is really very simple. By explicitly setting the cookie name in the authentication scheme (via shared components), you can link various applications in a single workspace so they will use the same cookie. This way, you don't have to authenticate with each application, but authenticating once will give you access to all applications with the same cookie.
As a simple example, you can create two applications in the same workspace. Set the cookie name (and the path and domain if you like) to the same value (e.g. myApexCookie) for both applications. After authenticating with one application, change the application ID on the URL, and you should be able to access the other application without authenticating. Trying this without the cookie, Apex will ask you to login again.
Things to consider
You should consider your authorization scheme carefully when implementing this SSO for ApEx applications. Each application, and thus each parsing schema, should have an authorization in place in the application, so not everyone can access all schema's.
When creating different applications, it would be a developer's nightmare to maintain the navigation for each application. So consider creating one menu, referencing the right application and page for each menu item and then sharing this menu with all the applications.