This paper develops the methodology, based on the segmentation method, to evaluate the production costs of two interconnected systems. The demand plane is divided into a grid structure, or segments, of equal size. Each segment contains the joint probability of the occurrence of load in the range of the selected segment as well as the first moments, or expected values, of load, or equivalent load, and residual tie line capacity for each system. The methodology proposed is accurate and computationally efficient. The method is applied to the evaluation of expected energy generation, unserved energy as well as production costs of two interconnected systems. The impact of tie line capacity variations, forced outage rate changes as well as joint ownership of generation is investigated.