Mixer inputs try to have impendances as low as possible to minimize distortion of the sound. Some mixers tout 'Very Low Impedance' or 'VLZ' as a feature of their mixers, which affects microphone inputs more audibly than line inputs. The fact that output impedance is always much greater than input impedance also means that most high-impedance outputs can power multiple inputs at one time. A simple splitter cable would be able to allow two inputs to receive a clear signal from one output, or possibly even more. The impedance would be so badly offset that the sound would be audibly distorted.
Most humans have two ears, duh.
In order fake the position of a sound somewhere in between, a proportion of the sound is distributed to each speaker. Stereo sound doesn't succeed in giving the illusion that the band is right in front of you, but it does heighten the aesthetic interest of music or sound recorded. One real benefit of stereo is that you can separate the positions of different instruments, so that each instrument can be heard clearly while being part of a whole mix. Pan, short for panorama, dictates how much of a single sound should be given to the left speaker and how much to the right. This represents which direction in the stereo image the sound should seem to be coming from.