All stats on the offhand weapon if of the same type as main hand buffs the main hand.
If you have 1HE in main hand and 1HE in off hand, all stats increases the global chance of crits, hits, fatalities, armor, combat rating, what ever. Like the Tigerswipe 410 combat rating (cold), that increases combat rating on main hand, since it will affect all hits, blunt or edged.
If you have 1HE in main hand and 1HB in off hand, the same thing applies, except if the 1HB carries a weapon specific stat of combat rating, like the Scepter of Return, has +175 Combat Rating (1HB) that will NOT enhance the damage done with your main hand, but only with the offhand hits.
This image shows what types of Combat Rating I have on my Conqueror.
http://i.imgur.com/NxJMTCV.jpg