Texture Tile Problem

Hi all, I''ve a problem concerning the atribuition of textures to a terrain: when i apply diferent textures in joined tiles (implemented by a GL_polygon) the line that joins the two polygons turns black and the joint becomes evident, i belive it''s a floating point problem, since the lines of the polygons are coincident, i''d like to know how to solve this, if you can help me, please do so! Best Regards, Anfernius