You can add 3 portals. If portal 1 is down user will automatically login to portal 2, if portal 1 and 2 down then user will login to portal 3. Make sure you added all lines to all portal. A line/user should be able to login to any portal using same username and password.
Let's say you are using Portal 1 for Live TV with VODs and no Series (Portal is running fromd different Server). And you are using Portal 2 for Live TV, VODs and Series (Portal is running fromd different Server).
If you want to provide two separate services to two different users/lines, make add user to only one portal related to his service. If you add user to both portals, by default user will always login to portal 1.
App will pick correct portal for the user.
You don't need to make any client or server side changes.