Follow up post.
I had a 5 hour day at the track on Sunday. That's 5 hours without a break and almost no shade. I did take a water bottle! This was a shakedown cruise to see how it all worked together after advice from you guys in multiple threads.
Start off, a Peak Design 30L bag. It allows me to carry two cameras, one with the 100-400 attached and the other with a 24-105. There's room for the Tamron 17-28 and Minolta 80-200/2.8 with the A to E adapter mounted. All accessories, including a small pair of binoculars if I want them, umbrella, poncho, and other stuff.
I ended up with a Peak Design Slide Lite strap for the 100-400 camera. I was a bit concerned about the narrow strap with the weight but it turned out to be no problem. The strap is connected to the lens' tripod foot and a Basic Peak Design bottom plate on the camera in a similar configuration to what
@Carbonman showed. The only difference is I wear it so the lens faces backwards. This puts the grip in a natural position for my hand and keeps the lens from banging into things. The tripod foot is rotated to the portrait position as per your suggestions. I thought I'd hate that, but it's well out of the way and I never even noticed it.
For the 24-105 camera I used a Peak Design (are you seeing a pattern here?
) Capture Clip with the Pro Pad Stabilizer on my belt. I wasn't so sure about this going in, but man is it nice! The camera is (relatively) easy to get on and off the clip (I'll get better at it) and the extra weight on my belt was negligible. Having said that, I'm not sure I'd want a bigger lens for this setup. I will add the Peak Design (sigh) cuff to this as soon as I get one.
So, one camera on a sling hanging on my right, second one on the clip on my left side. It was easy and fast to let the one hang and grab the other when I wanted a wider FOV. So much easier (and a lot safer for the sensor) than changing lenses out in that dusty, smoky air. I carried the backpack up and down like a bag, which was pretty light with both cameras and lenses out of it. One great feature of that back pack is no waist or sternum strap if you don't need them, and the shoulder straps are magnetically held to the backpack to keep it nice and compact. It was well worth the research and planning, and a big thanks to everyone here for your input and advice.
Yeah...I probably looked a bit geeky, but it was fast and convenient. I could change cameras at light speed. By the time I walked to my new position I had already made the switch and was ready to shoot.
Hey, at least I wasn't wearing a VEST!