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Draw multiple textures with additive alpha

I am drawing multiple textures during a transition from one image to another (slide show) 

// FIRST image with opacity = .4
glColor4f(1.0f, 1.0f, 1.0f, opacity);
glBegin(GL_QUADS);            
glTexCoord2f(...);
glVertex2f  (...); etc.
...

// SECOND image with opacity = .6
glColor4f(1.0f, 1.0f, 1.0f, opacity);
glBegin(GL_QUADS);            
glTexCoord2f(...);
glVertex2f  (...); etc.
The opacity of the two textures does not remain 100% during this transition revealing the background. I think I need to COMBINE or ADD the alpha but not sure how to accomplish this.
I have searched the list and google but can not find the solution. Any suggestions?

-Rich

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