NikeTalk is an ass, you don't need 2 sim cards. Not sure where he got that, just ignore him.
Just wait until the online account updates. It can take a day or longer. If not just call CSR and they should be able to help you out.
I'm at the same spot as you. I have reinserted the sim card to my iphone 3GS but it still has the $20 unlimited data instead of the $30. I see the option for $30 and selected it but it wont let me complete and tells me to talk to CSR. What next?
Just wait until the online account updates. It can take a day or longer. If not just call CSR and they should be able to help you out.