that is really a cute idea!

Haha if only HDR video was that easy.

You don't actually need to mask it, you could just process individual frames (pairs of frames actually, one from each camera) in photoshop using the "merge to HDR" function.

However, you'll find that the two cameras, even very slightly offset, will not give you a matching perspective. It would be like watching a 3d film without the glasses - everyone would have four eyes and two noses, no matter how well you match them together. Not to mention focus/zoom settings needing to be precisely matching. And you'd need to make the exposure changes using gain, not aperture, otherwise you'll end up with one camera having some objects within a wider DOF rendered sharper than the other camera.

For those using 5d cameras, there is a function inside the camera that can turn of hdr. This will help on backlit situation like mentioned here.

U'll get a d+ sign on the lcd whe its on. Just dont use it on normal lighting condition or u'll get unnatural looking images.