I don't think you can replace the 2 with 1. But there are cheaper aftermarket cats available. I am guessing around 350 USD. Just make sure you don't get CATCO. They look they were welded by Grandma Moses after she went blind. But there are other brands out there.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra