Posted for some EMEA love.
I went looking for Xbox Live Gold today and clicked on a link from EU due to my vpn's EU connection. I went to the US version of the site to see if the price was equally reflected at 50% off, and found that the 6 month option was not an option. Doesn't look like other EU options are 50% either, as this is the only one. It seems to only allow you to add one per purchase.
Did a search for it here, didn't find it, and couldn't make a purchase here in the states because it wants EU address/postal. If there's a workaround for the US, please post below.
[Note] - Posted this for EU folks looking for a deal. If it doesn't work for US bound addresses, which is stated in the OP, I'm sorry. I never claimed it would work for US, though I tried because it really is a good deal.