There is a thread on the TowerTalk reflector about stacking quads. I have never heard of it being done, but one of the participants pointed us to DF3KV and his stacked quads for 20, 15, and 10 meters.
It is an impressive picture and gallery. You have to appreciate the effort it takes to get these 6-element quads up on the tower and then geared up to operate. Especially when you consider that the tower is not that tall.