Beefcake,
First of all,
By loooking at your sig, you've got a e6600 which is not powerful enough for the x2. It works well with a 8800gtx, but the x2 is more than twice as powerful.
so you might be looking at a cpu upgrade in order to fully utilize your 4870 x2.
Second,
From what I've read about the x2 its only utilizing 1 core in windows xp.
That was back when catalyst 8.8 drivers just came out ( i thought that they'll fix that by now), so your card is not faulty.
I have the same card ( sapphire 4870 x2) with vista 64 bit and it works fine, but when i bought the card, i had xp and vista dual boot and i had the same problem - only 1 core was enabled in xp.
So I'm just tryin to save u the hassle of returning it, that is if u wanna switch to vista, but in the end it is your money so its up to you.
You can search for the "4870 x2 owner's club" in guru3d forums and post your problem there. I think that's where I read about the issue with xp and this card.
Oh and BTW,
As far as i know, you cannot disable one core so,no matter what settings u try, u should see the cores as "linked". It doesn't matter how u set the other options - catalyst AI and such.