For a three-piece acoustic act, a single monitor speaker might be enough to provide adequate stage sound for everyone in the band. A fancy (or widely spaced) setup might need two or even three monitors to do the job. These are normally all driven by the same signal, with the mix determined by the sound man at the FOH (Front-Of-House) position.
But when your band grows from four, to five, to six members and beyond, you are going to need a lot of monitor speakers. (Anyone who has worked with a six-piece or larger band is already familiar with the concept of a lot of equipment.)
It turns out that as the band grows, the need for different monitor mixes for different band members grows with it. It may be a matter of a performer properly hearing her counterpart who happens to be on the other side of the stage and can’t hear her over the general din either. Or someone playing a specialized instrument may need to hear specific cues from specific other players to stay in sync. In any case, here’s an easy way to set up a pair of independent onstage monitor mixes using equipment you are probably already using or have on hand.