This is a siamese cube. Siamese cubes are bandaged to the <U,u,R,r> moveset while a fused cube like yours is bandaged to <U,R,r>. I think the cubes joined at the corner are called something else. I wouldn't say that fused cubes any harder to solve than a siamese, just a bit different. However, a siamese cube is significantly harder to make than gluing two storeboughts together. Like I said, curse thrawst for his habit of calling things by the wrong name, people tend to listen to him. As far as I know, his "siamese" cube videos are by far the most popular, so it's understandable that you'd get mixed up if you watched his.
Calekewbs: No, Tony Fisher is correct, his cubes are bandaged to <U, 2U, 3U, 4U, R, 2R, 3R, 4R> which is just an extrapolated version of a siamese. A fused 6x6 would be bandaged to <U, 2U, R, 2R, 3R, 4R>.
Edit: If you are interested in building one, check out
http://www.speedcubing.com/ton/siamese.htm
