The demand for global telephony services for mobile users requires the use of constellations of non-geostationary orbits. The proposed S-PCNs are Iridium, Inmarsat-P, Globalstar, Odyssey and Ellipse. The first two systems propose to use a time division multiple access (TDMA) scheme and the others code division multiple access (CDMA) scheme. It is claimed that CDMA based S-PCNs can share the allocated frequency bands due to the interference resistant property of CDMA. Spectrum sharing must be based on defined interference criteria being satisfied. The interference events are based on the statistics of the C/I. For this a computer simulation was developed to model the dynamic nature of any constellation, and produces all the statistical values of the C/I to a mobile in a fading channel. The simulation includes constellations the results of proposed S-PCNs with multi-spotbeam antennas (MBA). We present the results of the investigation on the effect of totally overlapped band sharing on the bandwidth utilisation and resultant capacity and compare this capacity with that of a band segmentation approach. Computed values of the C/I in totally overlapped band sharing and segmented band sharing scenarios are translated into the system capacity. The results lead to specific criteria which must be satisfied for band sharing.