It handles arbitrary scaling and positioning and has an adjustable opacity contour for the blend (to deal with de-gamma and projector nonlinearities). It's designed to be released as a standalone app but I never bothered. You can hack it up and use whatever parts you need.
I attached it below and included a preference file, some alignment test images, and more photo documentation.
If you are a fan of GLSL shaders and Matrox D2G's, I came up with a solution for splitting a source to 2 screens and doing overlap/edgefade. The "tr.overlap3" shader will do the same for 3 screens (matrox t2g, etc.). The benefit of this solution is that it all works in a single window/context.