The first problem that comes to mind is the aluminum radiator. Swiftech's blocks have no internal protective plating, so you'll need to be very generous with the anti-corrosive in your coolant, or bad things are going to happen.
Next problem is power - running a pair of tec's off of your PSU really isn't recommended. Also, if you've got a multi-rail PSU, you'll have to be very careful not to overload individual rails, even if the combined rated output seems to be enough.
Thirdly, the 3870X2 uses different size bolt pattern from the 3870/50, so you're going to have a tough time getting the unit to attach properly, much less maintain the high clamping pressure needed to get the most out of a tec unit.
Fourthly, if performance is your goal (as opposed to doing this for the novelty), the cost of two MCW60-T's and the necessary adaptations that come with it are going to cost some coin. In fact, you can almost certainly buy a stronger stock GPU than any gains you'll get out of that card from the improved temperatures.
i7 2600K | ASUS Maximus IV GENE-Z | GTX Titan | Corsair DDR3-2133
Thanks, I didnt read the fine print on that radiator. I have an industrial 22 v psu dedicated for the tecs and the build was to test my mod skills more then anything. The rad is a problem however so I will not pursue this avenue.