Hi there..
i try to combinate 2 different objects but dont get the look i whant.
Take a look to the attachement.
How can i realize that (a+b=mixing between ab) ???

It looks like you'd have to use more than one path operation. I assume you've already tried them all, individually? Unfortunately I don't have time to expericment at the moment; but honestly I think you'd learn more doing it yourself (the trial and error).

Basically I'm thinking you'll have to have 2 or 3 duplicates of the red strip, and 2 or 3 of the blue, as well. But 1 or 2 of the blue duplicates would have to be edited (thus making them no longer true duplicates). I'm thinking one operation should give you red and white part of your result. Then just overlay part of the original blue. Yes, using Layers might be part of the solution, I'm thinking.

Although I'm sure there are others who may know just exactly what to do, or who have time to do the experimenting.