If you're layering in Photoshop anyway then try this:

Start with a black background, then set each layer above to screen. Screen only lightens whereas overlay lightens and darkens. So whatever's black in 1 layer won't add any light. Then you can adjust the intensity of each light independantly with the opacity sliders.

For even more fun, you can work with white light only and then adjust the colour of the light in Photoshop (I like using the Photo filter adjustment layer to warm up or cool down lights but I have a photo background so I find this way very intuitive).

I've had good results with just rendering each light independantly at 100% intensity and white and then colouring them in Photoshop this way and using the opacity as an intensity slider. I converted each layer to a smart object so that I could apply whatever adjustment layers I wanted to each light without having to worry about masking the adjustment layers (since the screen blending mode is applied to the smart object).

The big advantage of this technique is that you get to adjust each light after the render without having to re-render and your renders should go much more quickly with only 1 light per render.