Contrary to what's been posted I've found that as an R5 owner, with my 2nd body being an R7, I would 100% prefer to have a pair of R5's over the mixed pair. There are advantage to the mismatched pair, I don't disagree there. It's why I went for an R7.
But, the R7's additional reach over the R5 is minimal (vs. R6 it is much more significant) whereas the slight differences in ergonomics, menus etc. are far more noticeable in use than any reach advantage is.
Of course, economically, the R7 makes it a no-brainer. But if cost was not an issue, then I'd trade my R7 for a second R5. Yes, even for wildlife. Of course a pair of R5's is a good deal more $ than an R6 and an R7.
What really intrigues me about the R6II is what you get. Many people that own it are saying it is resolving like a 30MP 5D4, with slightly better noise handling. I have read similar about the R3. Couple that with the fact that the R6II like the R7, is benefiting from the AF improvements found in the R3, and it seems like a best bang for your buck FF camera. A Baby R3 for less than half the price.