  A generalization of the Complex Shell Model formalism is presented which
includes antibound states in the basis. These states, together with bound
states, Gamow states, and the continuum background, represented by properly
chosen scattering waves, form a representation where all states are treated on
the same footing. Two-particle states are evaluated within this formalism and
observable two-particle resonances are defined. The formalism is illustrated in
the well known case of $^{11}$Li in its bound ground state and in
$^{70}$Ca(gs), which is also bound. Both cases are found to have a halo
structure. These halo structures are described within the generalized Complex
Shell Model. We investigated the formation of two-particle resonances in these
nuclei, but no evidence of such resonances was found.
