The results of a detailed theoretical investigation of the phase behavior of A(m)-b-(B-b-A)(n) multiblock copolymer melts are presented for the special case of m = 20 and n = 10. The presence of two strongly different molecular length scales results in the formation of a large-length-scale lamellar structure of alternating A(20) and (B-b-A)(10) layers on cooling. On further cooling, the (B-b-A)(10) layers subsequently transform internally into small-length-scale layers of the A and B blocks: a lamellar-in-lamellar morphology. The final structure consists of an alternation of one "thick" layer and seven "thin" layers and compares favorably with recent experimental results of Matsushita and co-workers on slightly different systems.