What happens if an attacker and surprise defender is a tie but both stunned

The combat is a tie and the attacker is recalled — the defender keeps the battlefield and conquers it.

Since both sides are stunned, neither contributes Might to combat damage (CR 424.1.b), so no damage is dealt and both units survive — a stunned unit still needs its full Might in damage to be killed (CR 424.1.c). At Combat Cleanup, all units heal and the attackers are recalled because defenders are still present (466.1.a.2). With only the defender's stunned unit left, the defender establishes control and scores a Conquer if they haven't already scored that battlefield this turn (466.5, 469.1).

Stun doesn't change this: a stunned unit still holds a battlefield and conquers. The defender's unit stays stunned until end-of-turn cleanup (423.1.a.2), and the combat result is "No Result," so nothing like "when I win a combat" triggers for either player.