The twisting process to form the strand is very similar to braiding hair. 2-ply yarn consists of two strands twisted in opposite directions "Z and S" and then twisted together to create strength.
The use of 2-ply yarn means that you want to offer a quality product.
A sweater created with a single garment only has movement in one direction or "Z" or "S", so unless it is exquisitely designed, it will lead to garment deformation.
An example scheme of multiwire.
GAUGE: the number of needles used per inch.
The lower the "gauge", the more loosely knit the sweater.
The higher the "gauge", the more tightly knit the sweater is.