I think it partly depends on what 6 inch lift you have. If the lift has a spacer already then you wouldn't want to add another spacer.
I have a 6 inch stage 1 pro comp and a body lift right now as well. I was also looking into how to add more lift and I was told that the best way would be a new set of coilovers like the radflo 2.5, PRG UCA's, and a Deaver Leaf pack. This would give about 2-3 more inches of lift and can all be purchased from greg at PRG. If I remember correctly the pirce is something like $2000.
Ive heard of some people doing this, bolting a leveling spacer to the 6" lift spacer. Its deffinatly not the right way of doing and probably isnt the safest thing to do either as greg says right on his website "its not acceptable to stack spacers".
You can do it, but not with the regular leveling kit spacers, as they are too "tall".
What you need:
6" Suspension Lift Kit of your choice
PRG Upper Control Arms
Aftermarket Coilover (i.e. SAW/Radflo/Fox)
Custom Spacer, based on which lift kit you have, PRG can hook you up.
The amount of actual lift you can gain by doing this is dependant upon which lift kit you have. For example, it is well known that the Fabtech kit comes with the "tallest" spindles. This allows for about 1.5" to be gained by adding a spacer atop your coilover with the Fabtech kit. The ProComp/Trailmaster/Rough Country kits allow for about 1 to 1.25" to be gained. The Rancho kit has the shortest spindle of the litter, and I don't think anyone has even bothered with this setup on a Rancho kit.
Furthermore, member Yamatitan has had a custom piece machined to make his ProComp spindle taller/longer and use the same thickness spacer as what is used for the Fabtech, thus gaining an additional 1.5" w/ the ProComp kit.