The first image shows one circle overlapping another. They were drawn in a texture, with fbo, and blended with

glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,GL_ONE,GL_ONE_MINUS_SRC_ALPHA);
 

[attachment=15533:certo.PNG]
The second one shows the same stuff, but one circle is on textureA, and the other one on textureB (both handled by a fbo). Each texture was drawn on the screen, calling

glBindTexture(GL_TEXTURE_2D, textureA);
...
glBindTexture(GL_TEXTURE_2D, textureB);
...
 

[attachment=15534:err.PNG]

My question is: why the blending differs? How can I fix it?

For a better explanation on why alpha blending during runtime is wrong in the first place, check this out.

PS - what's in the alpha channel of the textures and the that of the active color when blending?

For a better explanation on why alpha blending during runtime is wrong in the first place, check this out.

PS - what's in the alpha channel of the textures and the that of the active color when blending?

Sorry, what do you mean?

Sorry, what do you mean?

Does the circle texture have an alpha channel? What's in it?

When blending, you'll usually want to use white color, eg:

glColor4f(1.f, 1.f, 1.f, fAlpha);

Or what is it that I failed to make sense about? :)