I'm guessing you could operate a walk-behind, or for that matter any multiple blade machine with a blade missing from now until the cows come home, at full throttle, with absolutely no ill effects. Each blade is independent of the other.
However, although I see the point in that the WB is a lot more powerful than a 21, and more suited to cutting the heavy growth, I'm also guessing that the turbulence under the deck is going to move a lot of the wood chips around, whether you are using one blade or two. Only way to know for sure, is to simply give it a try.
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