Did you save before activating the switch at the top? It took me a few tries, but with good timing, and starting off facing where the platforms will be, it is possible to jump up both before they disappear. There is a lever or button on the second platform which will fix them in place and cause a chest to appear on the ground near the entrance of the room, which you will need to drop off the platform quickly in order to reach before it disappears. At least in D2:ED it had a potentially decent legendary quality random item (good enough to try a few times for, but not enough to worry about if you can not get it with a reasonable effort); that may have changed in DKS.