GL_SAMPLE_ALPHA_TO_COVERAGE and depth testing I am currently looking into alternatives to my current, somewhat slow but exact, OIT approach. Specifically, GL_SAMPLE_ALPHA_TO_COVERAGE to convert the alpha value into a per-sample mask.

// draw opaque objects
glEnable(GL_SAMPLE_ALPHA_TO_COVERAGE);
glEnable(GL_SAMPLE_ALPHA_TO_ONE);
glDisable(GL_CULL_FACE);
// draw translucent objects


However, if multiple alpha-blended faces are behind one another (such as the back faces which I would want to be visible) I am only seeing the front most one. Is depth testing not done per sample? Can I force it to be done per sample? According to https://www.khronos.org/opengl/wiki/Depth_Test it should already be per sample.

Multisampling is enabled and works fine for everything else (my OIT shaders in fact do sorting per sample which might be a reason for the slowness). The FBO being drawn into is provided by Qt.
(Promoting comment to answer)
Do the faces have the same alpha value? That might generate the same coverage mask for the surfaces and, hence, only the closest one would be seen.