This is a quick tutorial on how to use the edge blending function in NTHUSIM to create a seamless image.
The first thing that is worth mentioning is that you do actually need to overlap your projectors. It won't be as wide as having projectors next to each other.
In the photos below I didn't have a wide angle lens, so you can't see the sides too well. I stood as far back as I could and that was enough to get the blend areas into view, but it is a nice wide screen! 
The first step is to make sure the projectors are overlapped enough. If you're not sure, it's usually better to err on the side of caution and give yourself extra overlap space. The stripe patterns will overlay each other by the time we have finished. Below is a photo of what overlapped projectors look like when only the four corner points have been moved roughly into place.
Once we have the stripes looking reasonable with just the corner points, it's time to add our first set of additional control points. Press F2 to add horizontal points. Then bring these in to start to match the curve. Notice how this will make it a lot easier to get the stripes a bit more aligned, but it will still be impossible to get them perfect. That is fine though, we want to gently massage the pattern into shape.
Once we have a better looking pattern, it will be time to add another set of horizontals (F2 key). This time the stripe will be much easier to get close. You'll probably notice that you can get the edges to match or the middle of the overlap to match but not both. That's fine - make the edges match for now. We will fix the centers up later when we have more control points. You should also add a vertical set of control points. This will make it easier because you can start to match up the white line. The screen I'm using here is spherical, so that is why the vertical lines look curved from where the camera is.
Now we're getting close! Lets add a few more horizontal points and see if we can fine-tune this puppy.
Here is where you can tinker quite a bit to get it well set up. But since we're more interested in the blending setup, I'll just leave it at that. You'll notice that we still have double-brightness in the overlap zones. This is easy to fix! we just scroll the mouse wheel to choose a profile that matches the projectors. In my case the profile was 12, but it depends on the output and color profile of your projectors, so just pick the one that looks good for you.
Great! now let's see what it looks like:
Some might say seamless!
Practice really helps, it took me a couple of minutes to set this up. It helped that the projectors were already in the right place and you may find that you need to adjust your projectors.
Here is another photo:
Hope this helps someone and good luck!