Technicaly a two-fer is an electrical Y that has female receptacles at the top of the Y and a male plug at the bottom leg of the Y; used to connect two instruments to the same circuit
In the drawing world a two-fer an take on a slightly different meaning. In general a two-fer is used to designate that 2 instruments are always going to have a common dimmer, circuit, or channel attribute. The ganging tool simply places two-fers, and you can put mutiple two-fers together to gang many instruments together. If you two-fer by channel, the channels will allways be the same, etc.
Multi-Circuit Instrument is an instrument, such as a striplight that is one physical unit, but has two or more circuits. The individual circuits of the instrument must be placed seperatly, then converted to a multi-circuit unit. If you move one part of the unit, the others will folow, but each part contains distinct information.