I see your point exactly. There seems to be an assumption that the camera will never fail, only the memory card. I would guess, in these times, it’s much more likely something goes wrong with the camera than a decent quality memory card. Therefore a backup camera would be more critical than one camera recording to two cards.Because in my experience failure at capture is close to non existent, failure of storage over time is guaranteed as all storage devices have a service life.
I have shot professionally since 1978 and I had far more issues with film processing issues/images lost than I have ever had with digital captures.
I know several pros who already use R6’s, the addition of dual capture won’t change their impressions of their cameras.
But you are still avoiding my key point, if your work is important enough to depend on dual capture why are you using a single R6 anyway?