Skylights, intensity and shadows

I’ve been playing around with dynamic lighted natures scenes and trying to understand something. My scene has a directional light (sun) and a skylight, both movable.

Here is an example image:


I’m having problem controlling the shadows without making the scene either to bright or too dark.

In the image above I would like the shadows to be brighter. But I can’t seem to achieve that without also making the tree really bright when viewed from the same direction as the direct light.
I’m using the props from the kite demo. Is this a bug, or am I doing something wrong?

PS: Is light intensity linear and interchangeable? (Is skylight:1 + directlight:5 is the same as skylight:3 + directlight:3?)