Thans for all the answers. The easiest way for me thats also in line with my thinking is to use two hemispheres, but I just still have to figure out how to map the image on the ground so it matches at the seams.

The hdri didnt show on the background object when I use the Environmental Map node - it shows with the normal image node but thenit isnt scaled around the whole scene.

@EldritchCellar I just had to realize that having different colors/values for diffusion and specular doesnt work in superfly, only firefly

thats a pity cause I think that using an additional diffuse light with image based lighting looks bad in most cases. but having no gloss at all also looks strange in some cases...

so what I have now is a single IBL light for arranging the scene, gving me a very coarse preview of the lighting and kind of light everywhere, and two hemispheres, and than I can switch the IBL off quickly for rendering.

its also a pity that HDRI bit depths are not supported in the IBL it seems, cause it seems to render faster and would be quite convenient to use.

I experiment with making an image based shader for the IBL that creates more dynamic but the result while being useful is not as good as using the lighting from the dome