Stacking projectors: Virtually every projector, if you have vertical lens shift, will allow you to virtually double brightness by putting two projectors, one on top of the other, and aligning them. Based on our measurements, a pair of G6550's stacked approaches 10,000 lumens. There was a time, 10 years or so ago when 10,000 lumen projectors were upward of $50,000! Stacking is a real plus for rental and staging, for major events. The staging company can use just one projector when its brightness is enough, or when needed, double up, gaining both reliability and brightness.
To stack a pair of Epson G6550WU projectors, you would normally use a vertical stacking mount, which is basically a cage to hold securely two projectors one above the other. with adjustments so they are both perfectly aligned with each other (same distance from the screen, both at a perfect 90 degree angle from the screen). The mounting system allows for that. Vertical lens shift then allows you to get the two images in what is essentially perfect alignment.
Two could also be "stacked" horizontally, using horizontal lens shift. They could even both be placed on a table top, next to each other. That may be easier, but since stacked projectors are mostly in a permanent setup, they are most often ceiling mounted, which is where the stacking mount makes sense.