Not having used any camera with dual SD slot before I can't answer with any conviction. I never shoot JPG (well, only when I mess about with time lapse) and I never shoot (important) video with a dSLR.
So at this point it's all a bit academic. For example, do we know how slots can be designated? Or are we just hoping that any option we like is possible?
Like others, I'm curious about the FLU tethering which if as good as it sounds (and isn't a pig to set up and subsequently unreliable) I can see it living in the wifi slot all the time.
Ask me again 6 months after I've had the K-3