Hidden 2h damage boost

Weapon 2h Test
Weapon Name	AR (1H + 2H)	Damage 1H	Damage 2H	Character	Enemy
Moonlight Greatsword +5	472	417	510	Int	NLR
Moonlight Butterfly Spear +5	333	273	342	Int	NLR
Uchigatana +15 (unbuffed)	420	269	346	Dex / Fai	PW
Character stats	Str	Dex	Int	Fai
Int	16	16	40	12
Dex / Fai	14	40	8	30
Enemies
NLR	Hollow in New Londo Ruins
PW	Hollow in Painted World

It was stated that weapons got a 2h attack boost because it multiplies the strength by 1.5

These weapons do not have strength scaling listed. Unlike strength weapons, these did not have an increased attack rating listed when 2 handed.

This is old news, but thanks for doing the tests.
All weapons with a STR scaling gets the two handed bonus of 1.5x strength.
But, most weapon's 2H attack hits harder than their 1H attacks, even though the AR is the same for both. 2H attacks also consumes more stamina to execute (and are usually slower/longer windup), so it's a trade-off.
On the other hand, the critical attacks (parry/backstab) of non scaling weapons should be the same 1H and 2H.

I just found out a few days ago when fighting Priscilla with the moonlight sword.

This was already know, there is an invisible multiplier for 2h regardless. The Str multiplier is just extra, and it is only on Str weapons that backstabs and ripostes do more damage 2h