I am a dual axe weilding player and noticed when fav-ing two of the same weapons only one appears in the quick menu.
Now occasionally very very random both do show up but only a for a short moment.

This is starting to tick me off so bad i now use a sword and axe.
That said a workaround is to enhance two of the same axes and name one "X" and the other "Y".

if you have 2 of (exactly) the same weapons does it not show up in fav menu as (weapon x 2). assign that to a quick equip direction on dpad and double tap that direction to dual wield. thats how it works for me, thats if i understand what you meant.

The way I do it is press R1 to equip it in the right hand then press L1 to equip it to the left hand, works everytime, you even see a little icon that says what hand it is equipped to.

Doesnt work for me, there is only one in my quick menu and the other one un-favs automatically when i exit the menu.