In brief, the method described here relies on the idea that a blend of two projectors should create a coherent image with regard to luminosity and colorimetry across the horizontal dimension of the image. If one were to split and overlap an image, with 60% on both projectors, the resultant image would have a band with twice the luminosity in the middle 20%. Simply halving the luminosity of both projectors on the innermost 20% of the blend would, assuming perfect colorimetry, yield an image that has the correct luminosity over the resultant image. In practice, with CRT projectors, this yield an image with three distinct color characteristics; one representative of the left projector, one of the right, and one that is the sum of both halves. Our eyes are sensitive to color change to such a level that no degree of manual calibration of the two projectors will yield a unified image with a simple halving of the intensity. To "blend" the images, therefore, takes a more complicated luma attenuation scheme, horizontally, across the overlapped region. Paul Bourke illustrates these blending concepts in "Edge Blending with Commodity Projectors," (http://local.wasp.uwa.edu.au/~pbourke/colour/edgeblend/
nVidia, in their Quadro drivers, supplies an application called "nvKeystone" that effects such a blend, over and on top of a hardware function that splits and overlaps the image onto two outputs of a dual-head graphics card. The parameters for this split, overlap and blend combination can be modified using the "advanced" tab of the "display" control panel in Windows, once the appropriate driver set has been installed. I assume that the mathematical basis for this software implementation is sound; but in practice, with CRT front projectors, the package does not yield a visually unified image. This may be the result of CRT projectors having a different gamma response than is expected by the algorithm, or may suggest that the projectors are set up differently with regard to full white/black levels than is assumed. nVidia doesn't supply any guidelines with their software, so all that can be said is that the result is close, but certainly does not yield a watchable image.
Without getting too far into the nVidia control panel itself, there is generally a zone, vertically, of increased brightness at the horizontal midpoint of the screen flanked laterally by two areas characterized by a slight deficit in brightness.
So without knowing what the expected projection device characteristics that nVidia expects are, I simply left the projectors set at setting that were as close to one another as possible and yielded a good image for a home theater environment, and then went to work on attenuating the blend.