Now that you have used two separate WDM MUX for the two OLTs, you can use a power combiner to combine the two signals from these two multiplexers. And on the receiver side, you can use a power splitter, followed by two WDM demultiplexers. You could have used just one MUX and one DEMUX at each side, but since the OLUs are not probably in the same place, I think you should do as suggested.
Regards