  We reveal underlying principles of nonlinear localization of a two-component
Bose-Einstein condensate loaded into a one-dimensional optical lattice. Our
theory shows that spin-dependent optical lattices can be used to manipulate
both the type and magnitude of nonlinear interaction between the ultracold
atomic species and to observe nontrivial two-componentnlocalized states of a
condensate in both bands and gaps of the matter-wave band-gap structure.
