Redering

HHHII! how can i avoid that the objects in my pogram will rendered on each other?? i mean if rendering two triangles with the same x and y values and diffrents z values and i set the camera roll around them on the y axis , in some point one of the triangles will be rendered on the another and i will get the efect that i can see one triangle througe the another triangle... hmm... somebody know how to avoid it? thanks