I think that's actually a different scenario, that favors the Big Stick, and, there's another, corresponding one that favors the little sticks.Being able to make 2 attacks instead of 1 is important because on some turns you will kill an enemy with your first attack and be able to attack another enemy with your 2nd while the comparison pc with a single attack doesn't get a chance to do damage except on the enemy you killed with your first attack.
The point I am making is that there is a similar scenario where the single attack PC is getting to do damage to another enemy while the two attack PC is still attacking the enemy the single attack PC killed the previous round.
What you've been focusing on, AFAICT is the likelihood of finishing off an enemy in the shortest possible time, that is, of not missing from the time you choose to focus on that target until it drops. Assuming the same chance to hit, that will favor the character who can do it in the fewest hits.
But, most over time, on average, they will kill the same enemies at the same rate. But, the Big Stick will kill more of those in the minimum time possible.
You don't even need to do any math, just think about it: Big Stick will also take longer than average to kill an enemy more often.
His DPR is the same, but his dead-enemies/round distribution is flatter.
Nothing to do with Overkill.