Take a trip down to your local track a see what most run and ask them. You may find that the Icharger Duo 406 or 308 will be in use most pits. They are capable of charger two batteries at once but most don't. They use the Duo to charge a single pack while utilizing the second channel to run a resistor bank for high current discharging.

There are a variety of chargers (Reedy, Protek, Hyperrion, etc)capable of charging two packs at a time but none have the spec and capability of the Icharger Duo.