  We consider the procedure of dressing of scalar and vector particles when
there exists the off--diagonal loop connecting vector and scalar propagators.
Instead of single Dyson equations for scalar and vector, we have in this case a
system of three equations for coupled full propagators. Using the $\pi-a_1$
system as an example, we discuss the physical meaning of this effect and the
renormalization procedure for coupled propagators. The considered effect of the
unitary S~--~V mixing may exits also in the "Higgs boson~--~Z~(~W~)" system for
extended electroweak models.
