The new Keystone feature is a great addition to the Present3D stereoscopic toolbox. First of all, it makes it really simple to align a dual projector stereo system, but then it also allows the projectors to be setup off axis, so not center of screen, but off to one side, thus keeping all of the equipment and cabling out of the audience space. This is really great for mobile dual projector systems, or even fixed systems where the projector mount doesn't give fine grain adjustment.
Stacked Dual Projector Setup
Even though you are going to use the Keystone feature for alignment, you do still want to get the best physical alignment of your projectors to one another and positioned so they are just slightly overscanning the projection surface, as this will retain the greatest image resolution.