Bus stops represent a key bottleneck that produces significant delays in travel times. In major cities, the width of streets is constrained by sidewalks and buildings. In this context, adding on-street loading areas in series is a valid method to increase bus stop capacity. A tandem bus stop is a linear bus stop with two loading areas on a dedicated bus lane where buses serve depending on the sequence in which they arrive at the stop since overtaking is not allowed. The objective of this paper is to provide an analytical methodology to help city planners to choose between conventional and tandem bus stops. This study focuses on the capacity gain of a tandem bus stop in the case where there are buses waiting in line. Current studies are based on simulation results only for particular cases. The model is applied to Barcelona, Spain, where the dwell time is described by a log-normal distribution according to the available empirical data. The results obtained in this paper indicate that a capacity gain of 30 to 70% can be achieved, which allows for substantial commercial speed increasing. If buses spend more time at the service position and the variability is low, the gain is greater. Our results suggest including in the Transit Capacity and Quality of Service Manual a more detailed description of the behavior of urban tandem bus stops, where the “effective number of berths” concept should be considered a variable (instead of constant) that depends on the dwell time characteristics.