Upload & Sell: On
Personally, I've always had just one and never really thought about getting another one. That being said, while the 6D is good at a lot of things, fast, small birds, it's not the best. I've thought about selling it( and still sort of do) and upgrading to the 5D4 to keep the simplicity of 1 body, but now I've been thinking of supplementing it with a 7D2.
As I see it, the 6D can be my do all body/low light and the 7D2 will be wildlife(especially with F8 AF) however, now I have added weight as well as bulk. The 5D4 would be a compromise of those 2 but also more pricey.
And yes, I know you pick whatever gets the job/picture done but what would you personally prefer?