Particle-hole excitations leading to two coexisting systems in the same nucleus are studied in the framework of an extended interacting boson model (EIBM). We apply the group theoretical concepts of the EIBM, a version of the interacting boson model where a distinction is made between particle and hole bosons, to the particular case of two coexisting systems in the same nucleus exhibiting the U(5) regular and the O(6) intruder configuration. One can allow for such coexisting systems through K-symmetry breaking in the ''vertical'' classification of particle-hole excitations. Furthermore, the ''horizontal'' classification allows to test the concept of intruder analog multiplets. Besides the description of these ''global'' properties, mixing calculations between the coexisting systems illustrate the important effects of ''local'' perturbations. Detailed applications are given for nuclei in the region of the shell closure Z = 50. (C) 1997 Elsevier Science B.V.