So long as the weapon in your second hand (your off-hand) is not used to attack, or is used with a full parry, you gain a +5 to your DB from that weapon.
You don't even have to have any skill in using that weapon to get the bonus to your DB.
The "spirit" of the ruling is that if you have something in your off hand that is large enough, it provides a tiny bit of DB (so long as you are NOT trying to hit somebody with it).