[Brightcolours]

Since getting the second body, it's become that much easier for me. :-) With the bodies of different formats, you're getting even more possible combos. I don't brandish two camera on every shoot but for the more dynamic and/or important ones, it's a great idea. If - in some other ideal world - I would've gone to the R system, I imagine using this 14-35 lens and a 70-200/4, though the lack of native, good and affordable intermediate option is a little concerning.

That 24-105mm coke bottle that Klaus is currently testing would run circles around the "standard zoom" you use on your current 1D, though. And it would be ok especially on an R6. Just don't forget to not turn the lens correction "off" ;-)
And of course, you can use any 50mm lens as intermediate between those two lenses too (even your 45mm f1.8 Tamron), and as lower light tool.

Just out of curiosity (and me forgetting), what is that second body you have?

Somehow I doubt a lens described by Klaus as a "coke bottle" would "run circles" around the 24-85 that I find completely sharp enough for work, if having noticeable CA at some focal lengths, plastic build and no IS.

My second body is the 80D; before that, I used a 650D (my wife's using it now with the 18-135 STM).

Sure the Tamron 45 could be adapted, but I spoke of the lack of "native" options like a good RF 50/1.4. Canon just can't get this FL/aperture combo right without drama, right? Somehow I doubt we'll be seeing it and with the apparent crackdown against Samyang lenses by Canon, I doubt we'd be seeing a third-party option either.

Klaus would not be impressed by that old 24-85mm Canon at all, on a modern body. The 24-105mm cheap one will beat it easily, that is how much lens design has progressed. 
I read that even the EF 28-105mm f3.5-4.5 II beats that 28-85mm on the borders (on FF). That lens was not all that sharp, but "pleasant", on APS-C 8-12mp cameras. Klaus only tested that 24-85mm on 8mp APS-C (350D). That was my first DSLR by the way.

So, yeah, I have no doubt that you would do quite ok with that cheap 24-105mm version, in general.
If you have that 45mm, why would you want a "native" one when using two bodies and seeing the advantage of using lenses on different formats (hello lens equivalence, my old friend) ?