Two of the most dependable konbini fillings, tuna salad and egg salad, share one set of bread slices in the tuna and egg sando. Rather than blending the two into a single hybrid spread, the standard approach keeps them as distinct layers or side-by-side compartments, so the eater gets both the cool oceanic salt of the tuna and the soft custardy richness of the egg in the same bite or in deliberate alternation. It is a sandwich for the indecisive and the generous-minded, and it carries a quiet logic: these two fillings already live next to each other in every convenience store cold case, so combining them is less an invention than an admission of what people were going to buy together anyway.
Construction is where this one earns or loses its keep, because it is really two sandwiches negotiating one space. The tuna side is canned tuna drained and bound in Kewpie mayonnaise, flaked fine but still legible as fish. The egg side is chopped boiled egg in the same mayonnaise, often with the yolks mashed loose and the whites left in soft dice, sometimes touched with a little sugar or mustard in the Japanese manner. The two are spread in parallel bands or stacked, and the bind matters on both halves independently: each salad needs enough mayonnaise to hold a mound without weeping, and the moisture levels should roughly match so one side does not soak the shokupan faster than the other. The bread is crustless and soft, pressed just enough to seat the fillings. A sloppy version lets the two salads bleed into a muddy gray-yellow smear with no contrast left, or skews the ratio so the eater gets a mouthful of all egg then all tuna with nothing in between. A good one keeps the two readable, the colors clean at the seam, every bite delivering the intended duet.
From here the variations multiply quickly. Some builds add cucumber for crunch between the layers, some introduce corn on the tuna side to triangulate toward the tuna and corn sando, and some lean the egg side toward a looser, almost sauce-like tamago salad. Each of those shifts the balance enough that it deserves its own article rather than being crowded in here.