I toyed with this earlier, and it's possible via a conditional branch in your action sequence for the attack skill.
Because the target action part of the sequence is repeated for each attack, if you wrap the whole thing in an if check that checks the targets HP, you can effectively not do the second (or any subsequent) swings if the target has 0 HP.
When I was investigating a bug report a few weeks ago, I'd forgotten I'd set this up on my action sequence in my test project, and thought my update had broken the offhand attack, but it turned out my first hit was strong enough to kill the target.
if (target.hp > 0) motion attack: user wait: 12 action animation: target action effect: target wait for animation else death break end