There is currently no way to do that exactly.
The modifier plugin parameters for dual wielding, offhand, and two handing a weapon can all be eval'd. By default, they're just a float number (0.75 for example), but you can use code in this box to make the number different based on a condition being met using a ternary operator:
(user.isStateAffected(21) ? 0.90 : 0.75
Something like the above would make the offhand do 90% normal damage if the user is affected by state 21, or 75% or not. These can be nested as well, allowing for different levels of offhand mastery, or ambidexterity, or whatever
(user.isStateAffected(21) ? 0.90 : (user.isStateAffected(26)) ? 0.80 : 0.75
I'm not entirely sure that's the exact code you'd use to do that, I didn't get a chance to test it, but I had set it up once a long time ago, in an old project, and it was designed for that use case. It's not exactly what you're looking for, but it can work for what you need.
The hit and crit bonuses are weapon specific. Any source of hit rate or crit rate that isn't on a weapon is given to both hands, and any that is on a weapon is only given to that one weapon. There is no way to change this currently.
~Ramza