I have Toro's also and wondered myself about double blades with our blade cradles. I have never had much of a problem with just high lift blades or gators, but I would like to try doubles just for the heck of it. My worry is LIABILITY. Blade comes off and hurts someone, we are doing something the mower was not designed for by the maker.
According to what I could find in the archieves, I have not read about any deck damages or mishaps, so I am willing to try myself. The Toro posts I could find made total sense to use a spacer, like a washer or lock nut, between the cradle and other blade. I like the lock nut idea, would help hold the "X" pattern, I would think. Only thing I did not understand is where this is at, second blade below the cradle or second blade ontop of the cradle next to the spindle! I guess it would not matter.
I do know that after I see how it looks and I can measure the blade and spacer I will be going and getting new longer bolts, only makes sense to me. Loss of thread means loss of holding power as well as added stress to bolt with added wieght and leverage on bolt ends.