One fibre is less expensive than two fibres. The physical construction of a fibre-optic network is expensive, the more customers are connected to a fibre optic strand the more efficient is its use. This is why Fiber7 uses only one fibre. In order to still allow a bi-directional communication, different wavelengths are used for transmission and reception.
In simple terms: in one direction we transmit red light, in the other direction we transmit blue light. In effect, the light is transparent and transmitted through a 1310nm wavelength and a 1550nm wavelength.