From the rules (
https://munchkinccg.game/gameplay/rules/#End_of_Game ):
"If there are no Heroes left in the game (due to simultaneous damage, for example), the game ends in a draw."
If this is a tournament that is being run by the full tournament rules (
https://munchkinccg.game/site-ccg/as...nt_rules_2.pdf ) you could use the Going to Time section to determine who gets the credit for the Win in the tournament bracket.