Dear Vamsi,
The DEMUX module in the industry have single input port. The same of the MUX, it has single output port. You may polarization multiplex the light ahead of the the multiplex or split the polarization before the DEMUX. However, each polarization could have the same or different wavelength band(s).
Using the components in OptiSystem, you should be able to split the polarization then have two DEMUX components for each polarization assuming each DEMUX have different wavelength plan for its filters.
You may build the device in OptiSystem and email it to me as zip file (ahmad.atieh@optiwave.com) and I would check it for you.
Regards,
Ahmad