Clamping alpha / colors when blending (?)

Now how do I clamp alpha and colors when drawing multiple textured quads over each other? I mean when rendering multiple object halos (that are GL_ONE/GL_ONE), if halos are drawn over each other, it results an overbright halo (twice the alpha and colors). How could I clamp those so it wouldn't become overbright, but all overlapped sections would remain max 1.0 alpha and max 1.0 per color? Like I said, my halos are 2D textures rendered as quads in GL_ONE/GL_ONE. Thanx!