Hey Peter,
About 6 months ago I created a design proposal to do something you are trying. The installation was for a clients penthouse. The throw distance was about 6' and the screen size was to be 18' tall.
I haver attached a pic showing how we were going to use stacked projectors and bounce mirrors to do this. The mirrors were huge. The blending was not going to work because of the steep angle difference between projectors.
If you look at the pic, you'll see that one projector was below the screen and the other was above. The good news was that we were able to use 1.8 lenses. With 0.8 lenses and better mirrors, it might be possible but nuts.