I agree it's unlikely.Dual memory slot in 6 series camera body is unlikely. Apologies.
This for many, many reasons would be a game changer. The biggest reason is, while all the bells and whistles of the 5 series are great, many people (like myself) would immediately turn those down in favor of a dual-slot 6 series camera and save the $1,200 - $1,500.
Canon has pushed people up to 5 series cameras on this feature alone since 2012. 6D body size was plenty large enough to easily support a second slot....we know very well that many would have skipped the 5D and done just fine with a dual slot 6D for their wedding and portrait shoots.
The only way Canon is doing this on the R6 is if the R6 is limited in a big way across many features. I don't see that. But who knows...with the camera market shrinking each year, maybe Canon is softening up finally.
Sony isn't offering 2 slots in that price range. Doubt Canon would up that. Unless they want to entice people away from Sony, but at the expense of the 5R? I doubt it. Canon didn't do that against Nikon's $2k range cameras....