Well, weak weapons, strong armor, and make sure they spar with very tough dwarves. Even then it's not fool-proof.
But I don't think it should be. If IRL you had 2 untrained people sparring with real weapons and no armor, and one was substantially more physically fit, it'd be dangerous. Violent sports see numerous off-season training injuries, that's amongst already trained athletes, with armor, minus weaponry, plus modern medicine.
Tamren, for your Order of the _____ idea, I'd think current nobles need to be fixed before adding more. Maybe even fix pathing issues so people could have more dwarves before slowdown since, as is, once the nobles start coming + fortress\royal guard, it is possible that you have no control over 1 in 4 dwarves -no need to increase that ratio I think. Otherwise sure, and I think the idea of an experienced trainer makes a lot of sense. Even with sparring as is, it's the untrained that injure each other a lot. Your ___ Lords are careful enough not to decapitate every poor recruit they spar with. (Usually.)
Also I don't think Legendary craftsmen should really be fighters. The incentive to make them fight is obvious, but that's partially cause they're so easy to get to that skill. I imagine being legendary would be closer to a life-long dedication to a trade\art and an exceptional talent at it. The idea of multiple-legendary skills ought to be very rare exceptions. How many people would you cite throughout history that've been worthy of being called "legendary" in multiple professions, let alone one.