In this work, we present a frequency transformation based dual-band filter design approach using scattering based real frequency technique. In the design process, conventional low-pass to band-pass frequency transformation is integrated with the simplified real frequency technique to construct double pass-band filters. The approach is particularly advantageous for designing matching filters between different termination resistances. As an alternative to a direct low-pass to dual-band frequency transformation, the use of dual-band mapping on a normalized band-pass prototype is investigated for efficient control of the passbands. Application of the proposed approaches for dual-band filters is presented by comparative design examples.