This paper discusses theoretically the formation of micelles in a bidisperse solution of diblock copolymers. The two populations of copolymer chains are of identical chemical nature but have otherwise arbitrary different block asymmetries. We find that these asymmetries, or correspondingly the relative ratios of curvature of the diblocks, determine the micellization scenario and the type of aggregates coexisting in solution. If the asymmetries between the sizes of the blocks of the two copolymers are not too large, the short copolymers are incorporated continuously in the large copolymer micelles and only mixed micelles are stable. If the asymmetries are larger, the mixed micelles always contain a finite number of short copolymer chains; mixed micelles and large copolymer micelles can then coexist.