griffon783, as far as I am aware there is nothing that tells us for which hand events occur (either hits or misses). I don't think we can even know for rogues whether the lower rate of poison proc's from the off-hand weapon are because its the one that misses more often, or that it has a modifier to its proc rate that functions similar to how it reduces the damage contribution from the off-hand. (Unless there was a blue post about it - anyone?)
So basically, your only guide on proc rates between you two weapons will be your poison usage (e.g., in 30 minutes, how many charges are left on the off-hand weapon compared t he main-hand if they both have the same speed). However, just to confuse the issue still further, keep in mind that proc-rates for most "chance on hit" properties (like enchants) are handled on a number-per-minute basis, not per-swing like poisons, making it still harder to figure it out.
Best suggestion I can give is to do some experimenting yourself. Get the cheapest enchant you can find that has a proc effect that shows in the combat log and put it on a weapon that you can equip in either hand. Then turn on your combat log (/combatlog) and with it in the main-hand record say 20 minutes worth of fighting. Logout, save your combat log, then log back in and repeat with the same weapon in your off-hand. Logout again and compare how many times the weapon proc'd in each case (i.e., search for how many times the weapon proc'd in each log).
You could post your results here even
-- Cirk