I recall reading somewhere that the OEM could use a wideband upstream and narrowband downstream, that the narrowband could be used to calibrate the wideband (sweep between rich & lean and find where narrowband is centered--then read wideband). In closed loop it should be darn close to correct coming out of the cat, so no real need for wideband after the cat. The cat should be ironing out the emissions. But pre-cat wideband can be more useful to the ECU. Never heard of them as air/fuel sensor though, just always "O2" or more recently upstream/downstream.
My truck is rapidly becoming a beater, but it's only at 160k. Original sensors. Probably keep for another year or two. But who knows.
$229 from Rock is $1 more than Amazon but should get here sooner, would like to install during our warm spell this week.