If you'd seen as many card failures as I have and also lost critical and irreplaceable footage for a documentary as a result, I think your priorities may be very different.
Jeremy,
That I may have been more fortunate than you (clearly) doesn't change the severity of data loss. Nor are my priorities when it comes to data storage, necessarily different to yours.
My point is that a second card slot, isn't the b-all and end all of data security. It may add an additional level of safety against a card failure (rare) or a fault of the cam writing to one card (rarer) but I wouldn't gamble the kitchen sink on it.
My go-to 'camera-case' backup setup is a MacBook Air with two external 512GB SSD's. I chimp often, just to check that the cam reads and writes to the card, alternate and offload files to the SSD's frequently when I'm not tethered (rare). Perhaps OTT in your book, not mine. To each his own, many degrees of data security, but in my book the second card slot is scant comfort. When I had the D800 I never used it. I'd choose a cam that that tethers rather than one that only offers two slots any day.
But I certainly accept that for some disciplines (sports in particular, pj also) a second drive is more 'de rigeur' than others. But even there, I'd suggest that real-time wi-fi transfer is more the way forward. We've seen the first steps in that direction, both from PhaseOne and Sony (amongst others) - I hope it accelerates.
In the meantime, all those OM-D, GH-4 and other MILC users, seem to be surviving on just the one slot. As I said in my original post - if I wanted the cam, for the cam's sake - I wouldn't be deterred by the single card, I'd look for an alternate safety strategy.
Best,
M
ps
Just saw your post - your dreaded Sandisk is my preferred marque!
Two slots won't solve our quandary ..