Alternator suggestions?

12 V is fine, but your battery should output at least 12.6 V. It is the battery's job to maintain voltage on the system, the alternator just recharges the battery. How do your fans get power? Next time both fans are on, read the battery voltage right at its terminals. You may be getting a voltage drop in the wiring to the fans. Best if that is dedicated wires right off the battery terminal (via a relay) and not shared with the rest of the car, especially the ignition system. If the battery maintains at least 12.6, go to the cable side of the terminal. You can easily get a drop due to corrosion.