I don't plan to lift my truck aside from what i did with using the tahoe springs in the back but what if he put the spacer at the top and another spacer at the bottom of the spring? Is that still dangerous or can they only be mounted in one place?
In my opinion, it's should not be done because -as mentioned, safety reasons.
Once a single lift kit isn't enough for you, you should then start looking into a whole "Body Lift Kit" where you modify the complete suspension. OEM parts (lower/upper control arms, tie rods, etc) can only stretch so far before they begin to fail and compromise safe road traveling. Stacking lift kits will make the OEM parts begin to reach-out too far and things can and will begin to break. And hopefully, not while you're traveling at a speed that can kill you or anyone else...
Moderators: If I'm wrong, then go ahead and delete or edit my post here.