In theory, there's an advantage to splitting the valving in half and each shock having half as much heat. But anymore you have to really search for lightly valved shocks. If you found the light-valve twin-tubes like came on it stock, you would spend as much on 4 as a pair of Bilsteins would cost - so why would you ever do it? Generally now you only find lighter valving in mid-to-high end shocks, which you will probably never overheat on a stock rig.
If you made a sweet suspension that could use double 5100-type shocks you would have already made some shock hoops or something. Then, for the cost of 2 5100/5150 shocks, you're getting into the nicer rebuildable shocks, which would be a better option.
So in short, there's a reason you don't see 2 shocks per corner anymore.