You can exchange and load your magazines in one of two ways: (1) If you're well practiced and your proprioceptive reflexes are what they should be, then, just drop one magazine and slam in the other without taking your eyes off the target. (2) If you have trouble with the above method, it's OK to cheat a little and use your peripheral vision in order to insert that second magazine. Some gunmen will extend the pinky of their strong hand in order to give themselves an additional reference point that doesn't require taking your eyes completely off the target.
(Watch Todd Jarrett swap magazines. Jarrett is often somewhere between method #1, and method #2. When he's really hot I've watched him move very smoothly through method #1.)