It has been shown time and time again that the stones have so much power, handling even one is a death sentence for most beings. Case in point:

• Holding the Power Stone almost kills Star-Lord, and he's half-Eternal.
• Using the fully powered gauntlet was very painful and seriously injures the nigh-invincible Hulk.
• Using all of the stones does indeed kill Tony Stark.

Even when Thanos is wearing the fully powered gauntlet, he appears to need to summon all of his physical and mental resources to withstand it. So when he's about to execute The Snap for the second time, how could he not have felt that the most powerful object in the universe wasn't on his hand?

Bonus question: How would the gauntlet have affected Thor if he was the one to use it instead of Smart Hulk?