Summation is implied by the default blend equation. The colour written to the framebuffer is
Cdst = Csrc*ksrc + Cdst*kdst
Where Csrc, Cdst are the source and destination colours and ksrc, kdst are the source and destination blending factors (α and 1-α in this case).
You can specify different blending factors for colour (R,G,B) and alpha using
glBlendFuncSeparate (requires OpenGL 2.0 or later). You can specify an alternative to summation using
glBlendEquationSeparate (also OpenGL 2.0 ); the options are add, subtract, reverse subtract, minimum and maximum.
Ultimately, if you have a large number of polygons overlapping a given fragment, and all of those polygons have an equal effect upon the result, you aren’t going to be able to “see” any detail. But in order to have an unequal effect, you have to decide upon the effect for each polygon. Normally this is done by sorting, so that nearer polygons have a greater effect than farther polygons. But that’s going to be costly if the number of polygons is large.
You might be better off just drawing wireframe. Lines typically cover fewer pixels than polygons so the amount of overdraw (i.e. the number of times a given pixel is drawn) will be lower.