help needed on openGL canvas overlay!

Hi,
I want to draw two overlaying images on the same openGL canvas, but in
the
same time I want to be able to handle these two layers independently.
For example,
if I need to update one of them, I don't need to redraw the other one.
So,
Is it possible to have two independent layers drawn on the same openGL
canvas?
Or can I overlay two transparent canvases on top of each other so that
the two
images can be superimposed but not interfere with each other?

Thank you very much!
Shao Chin