A "true dual" exhaust is two independent exhaust pipes from the manifolds/headders to the exhaust finishing point.
When dual exhaust is not a true dual is where the manifolds/headders meet at a common single pipe to a muffler that splits into two separate pipes to the finish, as found on every "cat back" exhaust system for modern cars and truck. Or even the more offensive inline 4 or 6 cylinder with only one exhaust bank that is split into two pipes at the muffler.
It is generally excepted for the most power to be generated from an exhaust it should be a "H" pipe system. A true dual system with a pipe about 2 feet back from the collectors to equalize pressure. Making the exhaust form an H with the piping