setup two wall system with one single PC and one quadro graphics card

[vrmdl]

I have no problem with stereo. The problem is that the floor image is not continuous with the front image at the edge. If I manually offset the floor image down for about -0.6, the image seems align with the front image quite well, i.e. the codes look like (the rest is the same)

C4 = -W/2,-H/2-0.5,-D/2
C5 = -W/2,-H/2-0.5,D/2
C6 = W/2,-H/2-0.5,D/2
C7 = W/2,-H/2-0.5,-D/2

The value "0.5" is obtained by trial and error. Is there any reason for this phenomenon?