One licence-key could be only used once at the same time. This meant you would need to stop at first the services on one server and could then start them on the other and vise versa which is only a workaround for some quick testings but not suitable for sensible development- or production-environments.
- No (see Marcus' explanation - only one server can be active at a time with the same license)
- Only Server B will stay on-line after migrating and testing? For help migrating a v11.xx server to v12, see: QlikView 11.20 - 12.10 Upgrade Overview For help moving your tasks and configurations to another machine, see "Migration to a New Machine" at the bottom of Upgrade and Migration ‒ QlikView