Can Polearm Master make a quarterstaff act as a double weapon?
Back in 3.5e, using both ends of a quarterstaff to attack was treated as using a "double" weapon, which counted for all intents and purposes as if the character were dual-wielding. 5e does not have "double" as a weapon property, but it does have the Polearm Master feat, which allows a character to make one attack at d8+Str and a second attack at d4 using your bonus action. Mechanically, this is identical to a character using a Longsword in one hand and a dagger in the other (which would require the Dual Wielder feat). My question is if a character wishes to wield a single quarterstaff, could they benefit from the Two-Weapon fighting style that is presented in some character classes and add their Str to the d4 damage when they attack with the opposite end of their quarterstaff?
The RAW seems to say that you specifically can only dual-wield by having a separate weapon in each hand, but is there any word on RAI for this?
Below are the relevant excerpts from the PHB:
Normal Two-Weapon Combat
When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand. You don’t add your ability modifier to the damage of the bonus attack, unless that modifier is negative.
Two-Weapon Fighting Style
When you engage in two-weapon fighting, you can add your ability modifier to the damage of the second attack.
Polearm Master Feat
You can keep your enemies at bay with reach weapons.
You gain the following benefits:
When you take the Attack action and attack with only a glaive, halberd, or quarterstaff, you can use a bonus action to make a melee attack with the opposite end of the weapon. The weapon’s damage die for this attack is a d4, and the attack deals bludgeoning damage.
While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
You master fighting with two weapons, gaining the following benefits:
You gain a +1 bonus to AC while you are wielding a separate melee weapon in each hand.
You can use two-weapon fighting even when the one-handed melee weapons you are wielding aren’t light.
You can draw or stow two one-handed weapons when you would normally be able to draw or stow only one.