That is too bad that you have already made that purchase. You can always order a coupon for the second set if you have. I cannot imagine they would ever offer a rebate on something you already purchased, they are being nice enough to help subsidize the initial cost ahead of time.
I purchased it on 9/20/2008 at target. I asked them about the program and they told me to order the coupon, but did not inform me that I would not receive a refund if I purchased it before getting the coupon.