  The phase behaviour of binary blends composed of A$_1$B$_1$ and A$_2$B$_2$
diblock copolymers is systematically studied using the polymeric
self-consistent field theory, focusing on the formation and relative stability
of various spherical packing phases. The results are summarized in a set of
phase diagrams covering a large phase space of the system. Besides the commonly
observed body-centered-cubic (BCC) phase, complex spherical packing phases
including the Frank-Kasper A15 and $\sigma$ and the Laves C14 and C15 phases
could be stabilized by the addition of longer A$_2$B$_2$-copolymers to
asymmetric A$_1$B$_1$-copolymers. Stabilizing the complex spherical packing
phases requires that the added A$_2$B$_2$-copolymers have a longer A-block and
an overall chain length at least comparable to the host copolymer chains. A
detailed analysis of the block distributions reveals the existence of inter-
and intra-domain segregation of different copolymers, which depends sensitively
on the copolymer length ratio and composition. The predicted phase behaviours
of the A$_1$B$_1$/A$_2$B$_2$ diblock copolymer blends are in good agreement
with available experimental and theoretical results. The study demonstrated
that binary blends of diblock copolymers provide an efficient route to regulate
the emergence and stability of complex spherical packing phases.
