This is what bamboola and I have tried to do, but the "delete" function won't work.
Here's a work-around that I tried last night. I set the expiration date to April 2014 and got an error message. I then set the expiration date to May 2014 and managed to delete one of the two credit cards. I presume that after May 2014, I will be able to delete the other credit card.