In the past I have used several Thermalright coolers for GPU's, they REALLY do a nice job and you can mount them in either an over or under configuration. So if you have Crossfire you can mount one over the back of the card and the other to the front. I have been told to watch the over the back configuration because if you have a large CPU cooler it doesn't work. I've only ever run this on one card. The last being my x1900xtx. This used to idle at 60C and peak at 90-100C. with the Thermal right, and 120mm fan (on a moderate setting) this idled at 40C and peaked at 60-70C. That's a 20C difference at idle for a VERY hot card. So I would recommend a Thermalright solution, I've never gone wrong with them.
However, my own opinion of VGA coolers is that you should just purchase one with an already upgraded, non-reference, cooler. It's saves you the hastle and cost, since a thermalright is about $50+, plus a fan. Just my thoughts. HIS has a REALLY good ICEQ cooler, and I wish they would sell it by itself.
Not sure about the x2 aftermarket. I have never seen a solution for this. ALTHOUGH, maybe you could use 2 HR-03 coolers if there is enough space. never heard of doing that though.