Whenever I see a B2 battle droid fighting, they are always using only their right arm. On the Star Wars Wiki, it states that the droids are equipped with dual laser cannons on their right forearm, but in the Clone Wars and in many pictures of B2 battle droids, they appear to have another laser cannon on their second arm. In one episode of the Clone Wars, a Jedi cuts off the right arm of a B2 battle droid and the droid attempts to lock its left arm into firing position. Also, in another episode, a droid popper was tossed at a B2's feet and the droid proceeded to shoot at the grenade with BOTH of its arms. Why don't the B2 battle droids attack with both of their arms, regardless of whether or not their left arm is a blaster or some other secondary weapon?
This is a link to a picture of a battle droid seen in the first episode of season one of the Clone Wars.
I'm not sure if there even is a correct answer, but my only thought was that the battle droids didn't have enough power in their system to function as well as fire multiple blasters.