We propose a new theoretical method to study hydrogen-bonded supramolecular liquid crystals. It is based on our recent theory of associating polymer solutions combined with McMillan's molecular theory of liquid crystallization. We derive phase diagrams of supramolecular liquid crystals consisting of two different species of molecules, such as dimers and trimers. In such phase diagrams, liquid crystallization and two-phase separation coexist. There are two different types of phase separation: one is caused by the first-order transitions of liquid crystals, and the other is demixing by repulsive interaction between the different species of molecules. As a result, multicritical phenomena such as tricritical. point, Flory's chimney type two-phase region, etc., appear. Comparison of phase diagrams of dimerized supramolecular liquid crystals with those of corresponding flexible polymers suggests that a new microphase separation transition may occur inside the layers of the smectic A phase.