(i have answered myself, problem solved)
If two middle-grey (0.5 value) images are used and combined in Overlay mode the image will get darker. How can i keep the same value and add the details of both textures?
Here is the ambiant occlusion map with a middle-grey:
And here i add another texture in overlay with a middle-grey this can be a bent map for the lighting or a curvature map, as you can see the image becomes darker and it becomes very dark if i increase the factor: