Edge Blending software allows you to connect multiple projectors so that they overlap and then set them up so that the output of each beamer is aligned and the brightness of the overlapping area is corrected so that the image looks seamless.

Quite a few of these packages only allow the output to be corrected in an openGL / DirectX environment.

Below is an overview of Edge Blenders that also allow the whole windows desktop to be projected over all the beamers.

Some of these can auto set up using a webcam.

Fly Elise NG Immersive Desktop costs EUR 69,- for non-commercial use, and EUR 150,- for commercial use.
These are guys in Eindhoven who built the software to power their garage built full motion flight simulator.

Vioso presenter Multi

Costs EUR 229,- / EUR 69,-

Warp Desktop

Allows 10 monitors per PC for GBP 330,-

Scalable Desktop

For up to 6 desktops.

Torsionsoft Torsionblend

Also allows projection mapping.

2 channels for CAD $799,-