So this is not the same as what is shown in the image you have attached. Do you know any papers on this matter to intoduce? In the meanwhile, I believe if the wavelengths of OLU1 downstream and OLU2 downstream are not the same, then you can simply use the MUX to combine. You can also go with the power combiner, too. Then, at the receiver you must use power splitter and then filters at each user the way I depicted before. It is as if you are using one OLU.