Hello @Geir3,
I've read before that if the second payment is refused you'll be offered the opportunity to enter a new card number to complete the transaction but I can't find the article at the moment.
For peace of mind it's probably best to contact Airbnb support.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.