If two creatures deal lethal damage to each other simultaneously, do both die at the same time?

When two creatures block and deal damage to each other simultaneously (during the combat damage step), neither creature actually dies at the moment damage is marked. Instead, damage is recorded on each creature, and then the game checks State-Based Actions (SBAs).

State-Based Actions are checked immediately after combat damage is dealt (and at other game events). Under CR 704.5g, a creature with total damage marked on it equal to or greater than its toughness is destroyed at that point. Since both creatures have lethal damage simultaneously, both are destroyed at the same time when SBAs are performed (CR 704.3).

This simultaneous death is important: neither creature 'sees' the other die. Abilities that trigger on a creature dying (going to the graveyard from the battlefield) will trigger for both creatures, and neither creature's death causes the other's — they happen in the same SBA check.

Example: A 2/2 Grizzly Bears and a 2/2 Centaur Courser attack and block each other. During the combat damage step, each deals 2 damage to the other. When SBAs are checked, both have lethal damage, so both are destroyed simultaneously and both go to the graveyard at the same time.

Note: If one creature had deathtouch, the same rule applies — SBAs are still checked simultaneously (CR 704.5h), so both still die at the same time rather than one before the other.