Using the kineme.net GL Tools "Grid Generator" and "Grid Renderer" patches, I constructed a little composition allowing two projectors to be overlapped and stitched.
The laptop output is connected to a Matrox DualHead2Go, and then to each projector. By tapping the left calibration key, I can place the four points for the corners of the intended surface for the left projector, then do the same for the right projector, and then adjust the edge blending.
(The ugly color-banding/quantization is an artifact of the interaction of the DLP projectors and the cheap point-and-shoot camera I used to capture this, and isn't actually visible to the human eye (mine, at least).)
This is loosely based on ideas presented in Paul Bourke's "Edge blending using commodity projectors". Instead of using his CoreImage filter (which runs really slowly on my hardware), I'm simply generating a gradient overlay and then applying a uniform gamma transfer function.