Kraum, Ludevic's Opus

Kraum's ability looks at the entire turn to determine which spell is a player's second spell. It doesn't matter whether Kraum was on the battlefield when the first spell was cast.

Kraum's triggered ability will resolve regardless of whether the first or second spell that a given opponent cast that turn has resolved, was countered, or is still on the stack. Notably, the ability will always resolve before the second spell resolves.

Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional 2 generic mana the first time you cast the other. A player loses the game after having been dealt 21 damage from any one of them, not from both of them combined.