Q: I have two cats on my Bronco, can I replace them with one?

A: Technically, no. Realisticly, yes.

Explanation: It is illegal to replace or modify emmisions control equipment with other-than-stock equipment. But the pre-cats are only there to reduce emmissions until the main cat gets warmed up. So yes, you can go to one cat and you might even burn cleaner with a aftermarket high-flow cat. But you will run the risk of not passing visual inspection.