Dear All
for stereoscopic projection it does not matter where the projectors are, if side by side or on top of each other.
for a stereo 3D projection with polarized glasses it is mandatory that both projector outputs are lined up as tight as possible on top of each other.
the best image quality is archived when stacking 2 projectors on top of each other and if possible the projectors pixel line up can be setup with the projectors lens shift.
If this is not sufficient you may use either Pandoras Box keystone correction or the projectors buil-t in one.
Please also make sure to use linear polarized filters in front of the projector.
The filters will need to be offset by 90degrees from each other in order to let you see only one projector per eye with polarized glasses.
Once the projectors outputs are aligned to each other all you will need to do is put on your 3D glasses and offset both outputs cameras x Axis slightly away from each other until you see the 3D effect.
The best way to do this is to use a rotating 2D layer or 3D object.
Best regards
Jan Huewel