I don't know based on the rulebook. The rulebook states that "as the last opponent’s Sanctuary falls, you are immediately declared the winner" (page 2), but it does not specify what happens if both players' last Sanctuaries are destroyed at the same time. There is no rule addressing simultaneous defeat or victory conditions for both players.

To answer this question, the rulebook would need to clarify the outcome when both players lose their last Sanctuary simultaneously, such as whether the game is a draw or if another tiebreaker applies.

Source: Core Rulebook (pp. 11, 2, 20, 22, 26)