color picking method to select primitive in opengl

Hello,

I am using color picking method to select a primitive in my application.. At present it is picking only single object.. I just wanted to clarify few doubts..

It is working fine when i click on a single object.. I mean if there are two objects lying on the same pixel.. And if i click that pixel, then i get only one object.. How can i get both the objects, using this method..?? Is it possible..??

THanks in advance