In Unity 3D if I click on an application in the launcher all windows of that application get to the front.
If I do the same in Unity 2D only last active one used gets shown. Is there a way to enable the same behaviour as in Unity 3d?
No, I don't think so. The behavior that you are describing for Unity 3D depends on Compiz (scale plugin). Using Unity 2D normally you don't have a compositing window managers like Compiz working, witch would be necessary for those kind of effects.