Not everyone, including pros, can afford to buy two of the latest generation of cameras. Many will place greater importance on buying extra (or better) lenses, and will often have one high end body, plus a cheap RP or R as an emergency backup.
But for those who *can* afford two hi-end bodies, it can make a lot of sense if they are identical. There will be many people who want a pair of identical bodies that can serve both for ultra hi-res and action, by simply switching resolution modes. The controls and operation of both bodies will be identical, eliminating any muscle-memory issues.
And, if one of those bodies fails or has to go in for service, you can still shoot any type of project with the identical second body. If you have two different bodies, that may not be possible.