Is your cat toast. If not what is wrong. Are you not passing smog. An aftermarket cat may work. I know there are Cali legal universal cats from magnaflow. They may have one that fits your car.
Adding more cats would have little effect unless they are close to the engine. They need to be very hot to make the catalyst work. Plus as W.Hunter says its probably not legal.

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Chris
84 280sl
82 300d euro