Diablo – How is follower DPS calculated

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.

You don't equip them. Your followers do.

Each follower (Templar, Scoundrel, and Enchantress) has a special equipment slot just for these types of equipment. You can access these slots through the follower inventory screen.

The inventory screen can be accessed by clicking on the follower icon at the top left of your screen or hitting the 'F' key (by default) if you want to access the inventory of the follower you are currently using.

The inventory of followers you are not currently using can be accessed by dialog options with them in town.