I've done similar stuff and posted here:
It was a 16m x 3m panel, I mapped during an university graduation cerimony.
I used two projectors and a MTH2G. You can use a tripple head and just use two of its three outputs. Or, you surely can use the Dual Head.
It was my first edge blending mapping. I don't know if this was the best way to accomplish it, but it was my way at the time.
This is my procedure:
I take the projection area (in meters, feet, whatever) and take it into a new file in photoshop.
Then I adjust the size proportionaly to better match my composition setup in width vs height.
Attention: you have to consider the blending overlaping projection pixels, so there's some loss at the sides.
Having this adjusted, fit it as mentioned above. You might and probably will end up with several pixel loss (area not going to be used) in your height or in your width, depending of your setup.
So, this is your working area, or so I called my mask area.
Anyway, I usually create my base content as twice the size of the final resolution, in pixels, just in case. Course, it is better to reduce imagery than increse, blow up pixels.
Well, now you have your working area (mask) and know how to create your video to fit it properly.
In Arena, create two slices and course you'll use each for one projector.
Make your projectors to project wider and heigher a few inches, according to the projection area itself, while being sure their projections are overlaping in the center, at least 15% of the frame, or the best matching for what you have specified before in photoshop. I usually take 128 pixels from each side, meaning that I will have 256 pixels less in the total projection width.
Now you just have to pull and push slice points to map your projection.
You better get 'twin' projectors, meaning same brand and, if possible, same model and bulb lamp lifetime. This will reduce your work on trying to match luminosity and colors. But there's no magic, they are never the same.
You will have to adjust color settings inside Arena, and perhaps on each projector as well. You gotta play with it to get the best blend possible. But I can tell you that I don't believe there is such a thing as perfection, concerning this.
That's pretty much it.
Sorry about the book ; )