This paper studies interconnection of DCS1800 and GSM900 dual-band networks. Two types of interconnection configurations for GSM services, non-overlap and overlap, are described. We propose analytic and simulation models to investigate the benefit of the overlap configuration. Our results explain how the radio channel capacities, the inter-system handoff failure rate, the originating call traffic ratio and the offered loads affect the system performance. This study indicates that with appropriate overlap configuration, the GSM dual-band network can significantly improve the quality of cellular service.