Krark, the Thumbless

To have two commanders, both must have the partner ability as the game begins. Losing the ability during the game doesn't cause either to cease to be your commander.

Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional {2} 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.

Krark's ability triggers at the same time as any other abilities that say "When you cast this spell" or similar, including cascade. Those abilities may be put on the stack in any order. Those triggered abilities are unaffected by the original spell being returned to hand, countered, or otherwise removed from the stack.

If you win the flip, but the spell that caused Krark's triggered ability to trigger isn't on the stack anymore (most likely because it was countered), the copy is still created.

You can't choose to pay any alternative or additional costs for the copy. However, effects based on any alternative or additional costs that were paid for the original spell are copied as though those same costs were paid for the copy.

If you copy a spell with targets, the copy will have the same targets unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for any of the targets, you can't choose a new legal target, that target remains unchanged (even if the current target is illegal).