You can churn two at a time. (if you can do both minimum spends in time, that is). And you don't have to immediately cancel your previous one. If Citi is tight on your total credit limit, it helps if you still have the older card, ready to "transfer the CL and then cancel" on the phone. If you cancel beforehand, the CL from the cancelled card may not be avialble to you for a new card for up to 6 months!

Here is the thread which discusses all that:

http://www.flyertalk.com/forum/miles...nors-visa.html