Thank you for your quick answer!!
We are using NEC light gray wide angle rear projection screen, dimensions 6 x 2.25m, and the company we ordered it from said they never had such a problem... Maybe the problem might be in the viewing angle of the projectors?
I've been using two fisheye lens projectors for rear projection (one connected through VGA and other through HDMI) to screen one image (edge blending I did through Resolume Arena).
Now, what is really strange is: when you look at the screen from the front, the image is fine, as you move to...