I recognize this problem, and even more urgent if the perfect solution, a car trunk, is not available
I use several bags, especially shoulderbags, as the backbags are only for 'transportation' and quit unworkable' although I have Lowepro slingshot' which can serve fine as it takes a lot of stuff, and the zipper can be opened with one hand while slinging the bag to the hips.
But for shoulderbags: (Bought them cheap for a few euro's at second hand stores, during the past 5 years)
1) Delsey 'gopix' - just zip open, and use. There is room for a body with medium size lens and extra lens.
2) a Dörr Adventure - with a very minor flap (not really in use) but with a zipper. That one takes two bodies D700 - average sized lenses, and lots of room for other stuff. Works great in exchanging rapidly to different body/lens
3) a leather Vanguard, a former film-bag. Takes a body with a long lens (70-200, 200mm, 300mm) - opens with two buttons only.
4) the same applies to a Samsonite, take two buttons to open up.
5-6-7) different shoulder bags to carry just one body with different sized lenses.
Number 1, 2 and 5 are used most frequent.
All of them can be combined too of course. Photo's can be made to visualize the bags and different situations