Multiple Transparent Windows

I work on a Sun Sparc Solaris workstation, which has Solaris 2.7 and Creator/Creator3D monitor adaptor.
I want to open two transparent windows over a non-transparent underlay window. I can do this partially. I mean, my two windows are transparent for the underlay window but they are not transparent to each other. How can I make them transparent to each other too? (I use glXGetTransparentIndexSUN function to make the windows transaprent.)
As an alternative to this solution, I tried to draw my images into two GLXPixmap and then show them one on the top of the other. But I cannot draw the images to the screen by using glDrawPixels!