A method for controlling an engine having multiple banks with separate catalysts is described. In particular, coordinate lean and rich operation between the banks is utilized. However, termination of rich operation may be different between the banks to prevent breakthrough of rich exhaust gasses due to lack of stored oxidants. In this situation, the bank that terminated rich operation is operated near stoichiometric. This minimizes breakthrough of emissions, while at the same time minimizing a torque imbalance between the cylinder banks. In particular, the torque imbalance can be further minimized by retarding ignition timing on the rich bank while the other operates near stoichiometry.