Dixie chopper has their "X-blade" technology they have been using on their ZTR's. I kinda liked the theory. The blades are curved in a design that they will seat together the right way when mounted and not move. I wouldn't ever just stick 2 blades on top of each other unless they were secured somehow. Either pined or welded, etc.. Balance might be an issue too, how do you balance that once it's welded..
I know some push mowers are using them. A friend of mine just bought a driven Honda, he said it looked like it had a 2 blade design. Loves the cut.