If you have two birthday parties to attend, one for your friend and the other one for your close friend, you should go to both at least make them both acknowledge your presence even though not for a long while. This way none will complain.
Close friend because he's your "closes" friend. Tell your best friend the situation your in and he'll probably understand why you can't make to his party. But before you go to your closes friend's party, have the courtesy to show up to your best friend party and give him a gift to show him that your still his best friend and that you care