I think the 6d+7d MkII combo only works if you think in two 'mindsets', the 6d for landscapes and the 7d MkII for long reach and wildlife. So pair wide lenses for the 6d and long for the 7d MkII. I have the 7d MkII and currently use a 70d for landscapes- just so I'm not changing lenses all the time in the field but I find too often I set out on a day and wind up being either in 'landscape' mode or 'wildlife (birding) mode' and fail to use the other that much. I don't know that there's a 'lens duplication' problem' if separate the tasks for each body. But I would never consider a body based on using a 1.4x with it on a continuous basis. (You can get the Kenko 300 series extenders to work with just about any Canon lens and their 1.4x is very good, but I wouldn't recommend using a 'short lens' with any extender as someone pointed out, the 85mm 1.8 is a very good lens and used is around $300- so why an extender on a 50mm?)
Thats the kind of thing Im talking about - you end up either carrying two cameras, or you end up only using one and not being able to use the other options, but at least wih 2 crop theres no lens mismatches. If you are in a situation or mindset for your shooting where that works its great - I too often find myself wanting a camera and two lens options, and the splitting became more of a pain then.
For me the main reason to go with the 7D2 is I already have a fair few crop lenses. I guess the 6D is cheap enough to try before considering a 5D instead, given a 7D is already in the picture, its the 6D/7D2 combo Im mostly warning against.