When you just run the lower drawer the upper fills

This looks like some problem in the draining system. Not sure if you are doing a new installation or the problem has just cropped up in a running unit.

The unit is very sensitive to the height at which you are draining out. Check the U splitter drain adapted is placed at the right location as shown in installation manual. If one unit does not get enough head to drain out it, the water could find a way back into the other unit through the U splitter.