On my 02 f150 4.6 I currently have true dual exhaust, headers, and a tuner running on it now.
My current cats are rotted out near the tops and clogged in th rears. I’m lookinb in to buying new catalytic converters but don’t really want to buy the factory direct fit since they’re more expensive, restrictive, quite, and they would have to be cut at the Y to fit the dual anyways Could I get away with doing one highflow on each pipe without CEL as long as I do the o2 sensors, one after the headers and one after the cat? My tuner allows me to turn off my rear o2 sensors if that would help at all.
Right now I have my rear o2s off and the only occasional code I get is the bank too lean which I always had come in every now and then even before the Exhuast and tuner, so I know the o2s off don’t cause a code themselves

 

That should be fine, but you really should put in a X-pipe if you run true duals. If you do trip a CEL, an option other than turning the rear O2's off in a tune would be the Gotts mod antifoulers.

 

 

https://www.f150online.com/forums/ex...-mod-pics.html

 

You can totally replace your OE cats with aftermarket hi-flows. Magnaflow part # 94136 would be a good choice. 1 per side, and definitely mount the front sensors in an appropriate location as they measure air:fuel ratio. I assume your headers are shorties?