I understand where you are coming from OP!
I've been considering this conundrum recently, how to carry three bodies at once and try and place some weight onto my / your hips away from your back. Thus far, I think the aforementioned SpiderPro holster is probably the best (and most expensive) solution that I've seen to accommodate this requirement. It can be configured to carry two cameras (one either side) and the plates allow for a quick draw from the belt.
Going slightly rogue here.... is the 400 2.8 strictly necessary for FoS? Could you get away with the 1-400 instead? Or do you need the low light capabilities - I love the 400 2.8, but it's so awkward to move about with!
In your scenario, I'd be looking at the following:
SpiderPro holster for two bodies
LowePro 600 / or similar backpack for the 400 2.8 / wa etc. Place the weight of the 400 on your back and protect it from swinging etc. also, I don't think that you'll be 'quick drawing' the 400 2.8. I've got a lowepro backpack that accommodates my 400 2.8 (just) but it's miles better than having it on a strap.
The SpiderPro holster will solve the issue of wearing a backpack with conventional straps such as the PD slide etc that interfere with the backpack strap etc.
Side thought - I'd love a manufacture to make a backpack harness for large lenses that's sort of like this:
https://www.amazon.co.uk/Scopac-Carrier-increased-comfort-telescope-Black/dp/B00A4Z2P5O
Where it's got a holder for the lens front and straps around the neck near the mount and allow you to carry the super tele's without the bulk for a whole backpack etc.