Since you know that you're already getting that second interview, wait and send the card. My advice though would be to call HR and get the names of everyone that is going to be at the meeting and your HR person and then send them all a card that has about one to two sentances in it after the interview like:
Thank you very much for taking time out to meet together last Thursday. I am looking forward to talking to you again soon.
I went and found the pretty little packs of eight of the same design of cards/envelopes because they will add up!
P.S. I'm about to send the same type of card for an in house interview that I just did out of the same deck that I bought 2 years ago, so keep 'em!