Known issue with Murky’s Battle Horn:
When both heroes die at the same time, you see a “Lost” screen, but it is recorded as a Draw, and in an Adventure, a Draw means you do not progress to the next boss level, you instead get to fight a boss of the same level again. In the case of Tekahn, that would mean restarting the fight with him. If your run ended, that that is another bug to add to the already acknowledged one.