In this paper, scaled boundary finite-element method (SBFEM) is extended to study wave interaction with two adjacent rectangular cylinders. In addition to the advantages in accurate and efficient representation of singularities and unbounded domains, the modified SBFEM shows distinct capability in dealing with wave diffraction including adjacent multiple structure interactions. Comparisons of wave runup and wave forces on each cylinder of the two adjacent offshore caissons with traditional panel method demonstrate a high accuracy achieved with excellent computational efficiency. Detailed results and discussion of the spacing effect on the hydrodynamics of the two cylinder system are presented. Design recommendations are made based on the maximum wave forces on both caissons.