In this paper, the theoretical model proposed in Zhang et al. (2019) is extended to compute the natural frequencies and modal shapes for two-dimensional moonpools with one or two recesses in finite water depth. In the framework of linear potential flow theory, the boundary value problem is solved by using a domain decomposition method, assuming the velocity potential at the outer boundaries to be nil. In particular, a new and efficient approximation method, double-mode approximations (DMA), is derived to estimate the natural frequencies and modal shapes for both piston-mode and sloshing-mode resonances. The present results using the derived DMA formulas are compared with the experimental results by Ravinthrakumar et al. (2019) and the solutions using the frozen-mode approximation (FMA). The comparisons show satisfactory agreement with the experimental data. In particular, it is shown that, in contrast to FMA, DMA can well predict the non-flat modal shape of the free surface at piston-mode resonance. Moreover, extensive parametric studies are performed to examine the effects of the moonpool geometry on natural frequencies and free-surface modal shapes.