Hmm, interesting. There's a lot going on there, so I think it would be a number of regions in the brain involved. Regions of the brain would maybe be:
1. Visual cortex, visual association area (occipital lobe at the back of your brain)
2. Primary motor cortex/premotor cortex (probably responsible for the actual aiming, but not the confused part)
3. Prefrontal cortex, this is the front of your brain and has to do with decision making. This region is going to decide what the primary motor/premotor cortex do, so something is probably misfiring here or just unsure about which target to shoot. Maybe the real problem is trying to prioritize the two enemies (due to the problem of one gun but two bad guys) but not having any information available to weigh one against the other (one guy having a bigger gun than the other, one of them with less health then the other, etc)