There are several approaches:
Continue with your actions in twitter, it will just work. If you click on any button or input text, or do any actions, bas will wait until action target will appear on page. If there will be none in a certain amount of time, bas will restart thread. So you can continue with your actions(posting, profiling, etc) and it will do everything for you.
You can wait while element, which is visible only for authorized user exists on page.
0_1461837482208_WaitForButton.png
If it exists on page, login and password is correct, if it doesn't, bas will fail with timeout and restart thread.
You can make custom waiter: loop which waits until element, which is visible for authorized user exists OR element, which is shown for non authorize user exists.
It should look like this:
0_1461838889045_CustomWaiter.png
And here is working project
twitterlogin.xml
You can check following pages, if you want to create custom waiters:
http://wiki.bablosoft.com/doku.php?id=how_to_wait_on_certain_event_on_page#custom_waiters
http://wiki.bablosoft.com/doku.php?id=is_it_possible_to_use_loops