Yes Richard I know that kit already.
The issue is not where to sit the lenses while changing but how long the camera is opened and exposed to the wind.
It is this lag that creates the problem as the dust comes in quickly and aderes to the sensor. Switch off the camera before removing the lens !
I have not yet changed lens using my plastic bag. It takes some time to do so and needs a surface perhaps just the lap... Not very practical, no...
Perhaps some body else will be able to drop a line on this subject.
You are doing well using two cameras with two lenses. I used to do that with a 20D and a 5D, one with 16-35 and the other with the 24-70. Oh... too heavy.
But now - I blame myself - I fell in the trap and bought heavy equipment. But I take a lot of pleasure to see my gorgeous images, to see how they come out so beautifully done by the combo.
Today I shot this one thinking on the "Devotion" project
Just a quick essay at f/1.4 !