To clarify, Dual exhaust would be the bottom one in this picture. The top one is not dual exhaust. The top one has dual tail pipes, but not dual exhaust.
I had dual tail pipes on a 4 cyl Mazda 3, but that's not dual exhaust. Also, just because an engine has 2 banks, doesn't meanit has dual exhaust if it merges at some point. To clarify that though, an "X" pipe or "H" pipe is not a merge, but simple a crossover to keep backpressure the same, it is the same reason that manufacturers put on an exhaust like the top one pictured, to make cylinder backpressures equal. But, the H & X crossovers work without actually merging the 2 air streams into one, where as that's what the merge (top picture) does, it makes 2 seperate streams into 1 stream and that is very unefficient, which is why aftermarket exahust exists in the first place.
P.S.: Picture is not nissan, it is Ford Mustang I think, but still relevant.