Unfortunately there is a bug in the Cookie Manager in version 2.3.1.
If the option "Clear Coookies each iteration" is selected, then all threads share the same cookies.
The Cookie Manager behaves OK if you don't select this option. If a single thread needs to login as multiple different users, rather than using the "clear" option, just make sure that the thread logs out of the application before trying to login again. (This is what a real user would have to do)
See Bug 44042
This mail message has details on some work-rounds.