I have this issue and can't see a solution anywhere (I have found a few people having the same problem). My Kensai 10/Mage 15 is dualing scimitars and I can't stand the humiliation done to me by Valygar, whose off-hand Thac0 is higher than mine! Valygar has 0 with the main-hand and 4 with the off-hand, while charname has 0 with the main-hand and 8 with the off-hand. I have taken of all the inventory except from the swords and it made no positive nor negative outcome.
Both Charname and Valygar have two weapons fighting on level 2. Valygar has katana proficiency in katanas level 2. Charname is the grandest of masters in scimitar wielding level 5.
My Charname Thac0 modifiers (as seen in inventory log):
Base Thac0: 11
Strength modification: -1 (that gives 10 Thac0)
Strength extra modification: -1 (dunno what that is, but it sums to 9 here)
Ability Bonuses: -3 (6)
Proficiencies: -3 (3)
Main hand Thac0 - Kachiko's Wakizashi +3: -3 (that's 0 so this one is correct)
Off hand Thac0 - Belm +2: 2 (Dunno if it's supposed to be +2 or -2, either way it should give Thac0 1 or 5, but I get Thac0 8!)
Where does this weird difference come from? Shouldn't I get a modifier 0/-2 from dual wielding? It looks like it's 0/-7 in reality, such a difference changes my enemy from a naked mage with Bracers AC6 into a tortoise in a scale armor!