overlay multiple textures from different pbufs


I’m trying to merge 2 clients (2 different pbuf) data on a frame. Lets say client1 (c1) has a unfilled triangle, c2 has a circle. I want to draw circle above the triangle or vice versa. For both clients, i am using glClearColor with setting alpha to 0 (to make everything transparent rather than the symbology). While i am drawing C2 over C1, I have to use blend functions. But when i use glBlendFunc(SOURCE_ALPHA, ONE_MINUS_SOURCE_ALPHA) it smooths the corners more. I mean the pixels whose alpha value is in(0, 1) are affected more which causes to make triangles thinner. Do you have any suggestions?