  Multicomponent quantum gases are ideal platforms to study fundamental
phenomena arising from the mutual interaction between different constituents.
Particularly, due to the repulsive interactions between two species, the system
may exhibit a phase separation. We develop a mean-field-based theory for a
two-component Bose mixture, which is equivalent to the Hartree-Fock-Bogoliubov
approximation, and derive analytical expressions for the phase boundary and
miscibility. The majority of existing theories, which are valid only for weakly
interacting Bose gases, predict that the phase boundary is determined by the
criterion $g_{ab}\leqslant\sqrt{g_{aa} g_{bb}}$ (where $g_{ab}$ is a coupling
constant between the components $a$ and $b$). We show that in the
Bose-Einstein-condensation phase ($T\leqslant T_c$) the system may remain in a
stable and miscible phase also for larger values of $g_{ab}$, depending on the
gas parameter $\gamma$ and temperature.
