In a small bowl, gently fold the rice vinegar into the cooked sushi rice and allow it to cool to room temperature.
Slice the sashimi-grade salmon and yellowfin tuna into long, uniform strips roughly half an inch thick.
Julienne the cucumber into matchsticks and thinly slice the avocado.
Place one nori sheet on a bamboo rolling mat, shiny side down.
Wet your fingers slightly and spread half of the rice evenly over the nori, leaving a one-inch border at the top edge.
Arrange half of the salmon, tuna, cucumber, and avocado in a horizontal line across the center of the rice.
Using the bamboo mat, roll the nori tightly over the filling, applying gentle pressure to create a firm cylinder.
Dab the top border of the nori with a little water to seal the roll completely.
Repeat the process for the second nori sheet with the remaining ingredients.
Use a very sharp, damp knife to slice each roll into 6 to 8 even pieces.
Serve the rolls immediately with wasabi paste, tamari, and pickled ginger on the side.