We pioneered the double sided banner in the early 2000s once it was realised how much better it was for branding purposes on a flag banner. The main goal is to keep the weight down as the heavier the fabric the more it will damage itself when flapping. To thsi day some competitors still use the three layer method with normal translucent fabric either side of an opaque centre piece.
Avoid getting sold double sided when it is a mirror image as below. These are printed on one side but the print is visible as a mirror image on the reverse side as below.
