I have set up systems with both single and dual fibres. Both can carry the exact same traffic. Major trunks would use 2 fibres, but the connection to the end customer is often a single fibre. Given the incredible bandwidth (~2 petabits/s) fibre supports, there's no problem with a using single fibre and different wavelengths. In fact, several wavelengths can be used, each supporting a separate connection. Coarse Divisision Wavelength Multiplexing (CWDM)
is frequently used to connect to customers. The wavelengths are separated with filters based on diffraction grating
I haven't lost my mind. It's around here...somewhere...