Hi Alitsu,
As of OptiSystem 14.0, the power combiner and the ideal MUX will now both simply add the powers. In fact at multiple wavelength input, the power combiner will act exactly like an ideal MUX. Of course, the power combiner can also add signals of the same wavelengths.
The previous method for the power combiner was based on a Wilkinson power divider/combiner model which we decided wasn’t appropriate.