Indoor wireless devices operating in 2.4 GHz and 5.8 GHz ISM bands have a wide range of usage area. However, mutual interference in neighboring networks degrades the system performance. It can also cause significant problems in secure personal communications on such wireless networks. Covering building walls with a band stop frequency selective surface (FSS) can be an efficient solution for such problems. Many available FSS structures are designed to have a single stop band. They are also subject to narrow incidence angle range and sensitive to the polarization of the wave. On the other hand, in some studies, FSSs with double stop band are designed such that the second stop band is to be the harmonic frequency of the first band. In this study, a dual layer frequency selective surface element geometry is introduced. This geometry is capable of at least 20 dB attenuation of incoming signals within the incident angle range from 0 to 60 degrees and for all polarizations. Another important contribution is that a periodic cell size of approximately one-tenth of the wavelength corresponding to 2.4 GHz is obtained. Besides, a new design methodology that enables almost independent optimization of each layer at its resonance frequency is also introduced.