Let's start with stacking. Virtually every projector setup (where the projectors have adjustable lens shift), can almost double its brightness when two projectors are properly stacked, instead of using just one. That makes a stacked pair of these Epson projectors roughly 12,000 lumens claimed. Stacking is a plus for rental and staging, The staging company can use just one projector when its brightness is enough, or when needed, double up, gaining reliability and brightness.
3D is not something a G6900WU does, out of the box. Rather, you stack two of these projectors together, and you install a different polarizing filter in front of each, and use those with 3D content, and passive glasses. (Don't forget, passive 3D needs a polarized screen.)
This approach is perfect for the larger venues that the G6900WU is likely to end up in. Doing active glasses 3D would be prohibitively expensive, say, in a university classroom with 300 students, whereas stacking two allows you to instead use very low cost passive glasses (just a few dollars a pair at most). The issues and savings are similar to 3D movie theaters which in the US, are all passive 3D. Even with the loss of brightness when viewing 3D, a pair of 6000 lumen projectors can produce 3D that's nicely bright on some seriously large screens!
Although we did not have two projectors, filters, etc., we have worked with another Epson projector stack in a review of the Epson W16SK two projector 3D system that is geared for 3D in the classroom. To learn more check out the W16SK projector review.