You have a Fighter with the "Tunnel Fighter" fighting style (bonus action to get attacks of opportunity without using reaction & reaction to hit enemy moving more than 5ft while inside reach) and the Polearm Master feat (attack of opportunity when enemy enters reach). The Fighter has a Whip (10ft reach) and a Quarterstaff (5ft reach, polearm).
An enemy moves towards the fighter, who has activated his Tunnel Fighter stance. The enemy enters the Fighter's Whip reach, then his Quarterstaff's reach. At this precise moment, is it true that the following can happen ?
- The enemy, now entering the Quarterstaff's reach, gets an Attack of Opportunity from the Fighter, who does not spend his reaction thanks to his stance,
- Then, as the enemy moved (more than?) 5ft inside the Whip's range, the Fighter can use his reaction to attack him a second time thanks to his stance.
My hesitation comes from the fact that the Tunnel Fighter's non-AoO reaction seems to demand more than 5ft of in-reach movement, which the enemy would technically not grant if moving from exactly 10ft away to exactly 5ft away.
The question also applies for the reverse scenario : an adjacent enemy moving outside the Quarterstaff's reach (AoO without reaction) and inside the Whip's reach (non-AoO reaction) at the same time. Note that this would not require the Polearm master feat to happen.
Anti-cheese notice : while the Polearm Master feat doesn't explicitly say that the reach-entering attack-of-opportunity must be done with the polearm, it would be logical to think it was implied, so the enemy shouldn't get another attack of opportunity when entering the Whip's reach (before strike #1). Else, this already strong combo would be ridiculous.