Optical coherence tomography (OCT) is a very high-resolution imaging technique whose resolution depends on source bandwidth. Improving resolution is an important topic of research in OCT. Thus, to improve resolution, the bandwidth of the source must be increased. Practical sources have finite bandwidth. Therefore, it is suggested to use more sources. Previous work expected that resolution will be inevitably improved without mentioning to what extent it will be improved and without any referencing to the effect of spectral separation between the sources. We study the resolution of a Fourier-domain OCT (FD-OCT) system based on two sources. First, we show to what extent resolution is improved and we show that this improvement not only depends on the spectral widths of the sources but also on spectral separation of the sources. Second, we show that in most cases resolution will become poorer and discuss mathematically the origin of resolution worsening and reveal the problems encountered in such a system. Third, we propose two techniques to overcome these problems. One of them is by shifting spectral interferograms and the other is by multiplying the spatial interferogram. Then, we clarify their advantages, disadvantages, major drawbacks, and limitations.