One theory is that this practice is leftover from back when pieces of sushi were made larger and then cut in half. But, there are also some toppings that are better to eat in pairs.
For example, conger eel tastes completely different when one piece is eaten with salt and the other seasoned with sweet sauce. Serving the piece with the head skin-up and the piece with the tail belly-up also offers different textures. Since the back and belly of bonito have different fat content, it can be better to order two pieces at a time in order to fully experience each of the individual qualities of the fish.
Many overseas visitors who aren’t used to eating fish have an aversion to fishy smells. This is actually the smell of a substance called trimethylamine and is generated by the breakdown of the umami component called trimethylamine oxide found in large amounts in fish by bacterial growth. The smell also gets stronger with the generation of ammonia as more time passes.
Bacterial growth can be controlled with refrigeration so toppings at sushi restaurants are kept cold. Trimethylamine is an alkaline, so smells can be eliminated by washing with vinegar, which is acidic. It is also possible to kill bacteria on the surface of the fish by soaking it in vinegar, reducing the number of bacteria. Basically, sushi restaurants are constantly taking measures to prevent bacterial growth and avoid fishy smells.
Sushi was originally a food eaten with your hands. Even, for example, if you are in a prestigious sushi restaurant in Ginza, you can still eat with your hands. You can actually use your hands to eat the ginger too. Restaurants that prefer you to eat with your hands will provide an extra, smaller towel for cleaning your hands between sushi, along with the normal hand towel. Even then, feel free to use chopsticks if you prefer.
A purchase of raw tuna costs at least JPY 30,000 per kilogram. Furthermore, good tuna is judged not only by taste, but appearance is also highly regarded.
The surface is gradually oxidized by letting it sleep (mature) and the sushi chef makes sure that parts are cut of as they change color, when the timing is perfect for both the taste and appearance. In other words, skin is taken from the freshly purchased tuna, the meat of the fish darkened by blood (the blackened area that can’t be used as sushi toppings) is removed, the parts that have changed color are shaved off and then only the remaining, best parts used as toppings are left.