I think I would stay away from putting double blades on that machine. I just dont think that it would have enough HP to do what you want, and will put unnecessary stain on the engine. With a large machine, and more HP you are able to do that.
It depends on which rider you get. What are you going to get, a Dixie Chopper, Exmark, Toro, or equivalant? If so, you should have no problem. If your rider is the same brand as your 21", I wouldn't advise it.
I wouldn't try to use doubles on a 21" unless you get the mulching blade that looks sort of like double blades. Most 21" has a 2 bolt or cradle type mount.
along time ago i tried the double set up on a 5hp 22. It would do a great job if the grass was short but if it were tall and the engine bogged down it took too long for it to pick back up. Too much time lost-the single works fine for a pusher.
I have used double blade system before, but it was under manufactures, spec. Honda's uses a double blade system when mulching.
In 25 years of mowing grass, I have never seen a double blade system other than the honda. The little 5hp 6hp engines can't handle the extra blade. Plus, on just most 21" commercial models, they only have a 1/4" clearance from the bottom(rim) to the blade.