The Wilson 859922 Combiner/Diplexer is a coupler which allows you to either combine 800MHz/1900MHz into a single signal (or vice versa). It is most often used to combine signal from two single frequency amplifiers to be fed into a dual band amplifier. It can also be used to split signal from a dual band amplifier into 800MHz and 1900MHz components. This could be used if you have two areas with people on different cellular networks, although this is rare.
Equal amounts of signal sent to two antennas for similarly sized areas