Here is an answer I found at the web site of a fiber optics manufacturer
A common question we receive is whether a 50/50 beamsplitter can be used in reverse, to combine the
signals from two sources, thereby combining their output powers. Unfortunately beamsplitters are reciprocal in nature, meaning that they
perform the same operation in both directions. Since the light from the input port on a polarization maintaining splitter will be split 50/50 into
the two output ports, light sent back along either one of those ports will also be split 50/50. In a one by two splitter the other 50 percent is
simply lost inside the coupler housing. In order to combine signals with low losses the two input signals must somehow be different: for
instance light of two different wavelengths or light of two different polarizations can be efficiently combined.
And also this:
Wavelength Division Multiplexers (WDMs), also known as wavelength combiners or splitters, are used to combine or separate signals. We offer 2-wavelength WDMs for visible/NIR or IR wavelengths, 3-wavelength WDMs, and polarization-maintaining WDMs. Infrared multiplexers are an ideal solution for combining pump and signal powers or for combining or separating telecom signals. Visible/NIR wavelength WDMs are commonly used for multi-color displays, sensors, and microscopy.