[Peter Koenen]

For those who are still searching for an answer, I got this solution.

Create a main Comp where you put all your content.

Then make two comps for the cameras. In those comps you put your main comp and a camera layer.

Be sure the ‘Collapse Transformations’ option is selected (gear icon) so the 3d layers from the main comp remain 3d in the camera comps and you can animate your camera in 3d.

Now make a Mix comp where we will mix/switch the cameras. Put the two camera comps into the mix comp.

Now your cameras are two comp layers so it’s easy to switch cameras with opacity, or use both cameras at the same time and animate them.