Because of the massive amount of spray that Dual Gunners are capable of it would be a bad idea to increase their damage. It makes sense to have 1shot headshots for the other classes because it is simply a lot harder to headshot with them at all. The spray from a Dual Gunners guns makes it nearly impossible to NOT head shot someone in a quick burst and I even find myself getting random headshots with it from across the map.
Ontop of that the Dual Gunner's can heal themselves and are capable of jumping repeatedly without effecting their accuracy in any major way. Also, with their high speed they can quickly maneuver and avoid incoming enemies while laying down quick, unavoidable fire to their enemies. Another advantage is that though it may take them quite a few shots to kill an enemy their bullets do still effect the opponent by slowing them down and throwing off their aim. Put all that together with their small figure which is hard to hit and I would say that they are equal to any other class if not better.