I constantly shoot with both the 80D and 6D2 and most often can't tell the difference at a glance with image quality. Once I get to pixel peepin the 6D2 has the edge. I can always tell the 80D though from the 6D2 at a glance by the auto white balance. The 80D images always look a touch cooler than the 6D2's. The 6D2 almost always nails the white balance perfectly in auto white balance mode.I just compared it to the 80d as is seems to be the closest crop sensor variant in the canon lineup as it seems to be the most logical - they seem to match the most (like the 7d and 5d series seem to match closely in the pro line up). I could be wrong with both of those comparisons but they share the bodies from looks. For the cost, I would have though the 6dmk2 would have had the same features as the 80d, predominately the headphone jack and all-i video options in the case as a vlogging camera.
These two cameras pair perfectly in my opinion and both are extremely good cameras.
My 80D almost always has my 70-300mm F/4-5.6L lens on it and my 6D2 almost always has the Tamron 45mm f/1.8 lens on it.
(my 16-35mm f/4L is always in my bag and ready for the 6D2 if needed)
This set up for what I do most often is perfect, I very seldom have to change a lens; I just reach down and grab one camera or the other