Bring a large saucepan of water with a steamer basket to a boil. Put the snow peas in the basket, cover and cook for 2 minutes. Remove the basket and transfer the snow peas to a bowl of ice water to cool. Drain and pat dry.
Add the shrimp directly to the saucepan of water and return to a boil; cook for 2 minutes. Drain, then plunge the shrimp into a bowl of ice water to cool. Drain and pat dry.
Slice each shrimp in half lengthwise. Cut the snow peas diagonally into 1/2-inch pieces, discarding the ends. In a large bowl, toss the shrimp, snow peas, radishes and scallions.
In a small bowl, whisk the vinegar, both oils, the ginger and 1/2 teaspoon salt. Toss with the salad right before serving; top with sesame seeds.