We predict the thermodynamic stability of mixed micelles of various morphologies (spheres, cylinders, lamellae) in the solution of AB and BC diblock copolymers in a selective solvent. The B blocks are assumed to be insoluble and form the core of the micelle, whereas soluble, incompatible (with each other) A and C blocks form the corona. A homogeneous mixing of the A and C blocks in the corona (mixed micelles) is possible if their incompatibility is relatively weak. Otherwise, a mixture of "pure" (ordinary) micelles consisting of either AB or BC copolymers is stable. Phase behavior of the solution, which depends on the relative and overall lengths of the blocks, and on the surface tension coefficients, is analyzed.