i almost make my little cell shading engine work, i can draw my model with the shading on it. Now i want to put the shading over the same model but who is texturized with a 2d texture. I can''t make it work, when the 2 models are exactely at the same position, i only see the last one that is draw. Is there a way to compute the shade with the current color?

I imagine you have the 1D texture for cell-shading and a 2D texture for decal. right ?
Then you have two possibilities : multitexture or multi-pass. which one are you using now ?