I was trying out the dual-wielding in Diablo 3, when I noticed really strange. When I equipped my off-hand, which has 0.5 less damage than my main-hand. My overall damage actually went down. So my question is, how does the dual-wielding mechanic work for calculating damage? It seems rather odd that my overall damage should be more with one weapon than with two.
Dual-wielding does not mean you attack twice as often. There is a slight attack-speed benefit, but this is apparently being offset for you by the lower damage from the offhand weapon.
According to this thread on the forums, the formula for number of attacks per second is
((Main Hand APS*1.15) + (Off-hand APS*1.15))*1/2
Which means that if you had two identical weapons, dual-wielding would give you a 15% attack speed bonus.
The main reason to dual-wield is that you get the bonus attributes from both items. This doesn’t mean much early on when attributes aren’t very strong, but is a big incentive later on. However, compared to a two-handed weapon that does more DPS, there isn’t a clear winner. It will mostly come down to whether you’ve found better one-handed weapons or two-handed, and what your preference is for faster attack versus stronger attack.