How to set window transparency?

Hi All,

I am very new to SDL, I need help.
I am creating two windows of different sizes and displaying one window on top of another.I want to make part of top window transperent so that images rendered on the back window visible.

Can anybody help me to do the above operation?

Chanabasappa C

why would you need/want 2 windows for this? could the same thing not be accomplished easier by using 2 render targets that are then superimposed (with the top one being alpha modded)?