code 10 is for main hand, 20 is offhand, so 30 allows both. this should be all that is needed.
If that is not sufficient, try setting the item size (weaponsize column) to large (4). This will keep it out of the offhand in many cases due to internal mechanics, but may restrict small characters from using them entirely (unsure).
I don't know if that will guarantee that the offhand will not accept a weapon. What your character can wield is based on the appearance lines for that character. When I was building large and huge creatures for last months' CCC, I was able to wield two two-handed weapons on one creature.