Aye, in those circumstances, it would seem a grrat ability. I'm not sure if Divinity uses typical D&D defined rounds so I'm in muddy waters; however, if it did, and your character were to be rated at having 2 1/2 attacks per round, adding +1 attack to that to make it 3 1/2 would balance things more evenly in the long run.
Nonetheless, if there is no concept of 'round' in the game beyond a recuperation time, then what we're really trying to comprehend with ambidexterity is not another attack, but a faster attack speed, ie, a lesser recuperation time. Follow me? <img src="/ubbthreads/images/graemlins/silly.gif" alt="" />
So maybe,
Ambidexterity
level 1: recuperation time -10% (lvl 1)
level 2: recuperation time -20% (lvl 5)
level 3: recuperation time -30% (lvl 10)
level 4: recuperation time -40% (lvl 15)
level 5: recuperation time -50% (lvl 20)
This is dependent on the engine being able to handle decimal-like numbers for a recuperation time, of course, for weapons that already have low speed numbers.
Your offensive rating, I think, shouldn't be further enhanced with any skills, beyond those found in the warrior's skill tables for separate weapon types. So, for instance, you would have to make a big investment of points if you wanted to not only enhance your offense for a sword, but also bring a mace into your other hand and use it with a similarly raised offensive rating.
Edit: i just read over my post and realized this wouldn't be dependant on using a second weapon <img src="/ubbthreads/images/graemlins/rolleyes.gif" alt="" /> But... anyway, the concept still holds true for a skill to increase attack speed, lol.
