The functionalization of various substrates through the reduction of diazonium salts is a widely used and expanding method, leading to the formation of highly robust functional surfaces, which is essential for specific applications. To further broaden the applications of functional surfaces, it is relevant to control the bi-functionalization with two independent and simple molecules. Some strategies, which differ according to the objectives sought, have been proposed to lead to the formation of such surfaces. The main objective of this review is to compare the strengths and weaknesses of the methods already described, with a specific focus on their ability to control the layer composition.