Phase diagrams of systems with two coupled order parameters

Within the framework of an exactly solvable model, which takes into account the interaction of fluctuating modes with equal and opposite momenta, we consider phase diagrams in systems with coupled scalar order parameters. We show that, in agreement with the renormalization group theory, the fluctuation interaction can split the continuous disorder-order transition into two phase transitions of the first order. Moreover, the transition may occur into the anomalous, from the mean field theory point of view phase. The effect disappears when the fluctuation interaction is suppressed.