There are five overloaded TankDrive methods in RobotDrive.h. One of them takes two joystick pointers. Another one takes two jockstick references. So TankDrive(stick, stick2) would work.
Code:
void TankDrive(GenericHID *leftStick, GenericHID *rightStick);
void TankDrive(GenericHID &leftStick, GenericHID &rightStick);
void TankDrive(GenericHID *leftStick, UINT32 leftAxis, GenericHID *rightStick, UINT32 rightAxis);
void TankDrive(GenericHID &leftStick, UINT32 leftAxis, GenericHID &rightStick, UINT32 rightAxis);
void TankDrive(float leftValue, float rightValue);