Hi Shahbaz,
Since the system is using the WDM access scheme, I assume the two signal you are talking about have different frequencies (or wavelengths). So you can simply use a multiplexer. However, you can use power combiner either. In this case, at the receiving side, you need to use a power splitter and then for every user, you have to use an optical filter that only passes the wavelength corresponding to the user. You can take a look at a recent discussion about this issue here:
optiwave.com/forums/topic/twdm-pon-2/
Also, you can upload your osd file if you needed more specific help.
Regards