Frequency and duration evaluation in interconnected power systems involves the integration of both parallel and series elements. The paper presents a comprehensive technique for performing this task in the form of an integrated algorithm. This procedure is then extended to utilise a correlated load model developed from the chronological occurrence of load values in the two systems to assess the integrated system adequacy. The method to incorporate load correlation is fully described, and examples based on small hypothetical systems and the IEEE reliability test system are used to assess the correlation severity.