In saucepan combine sugar, 1 envelope gelatin, and 1/4 teaspoon salt; stir in 1 1/4 cups water, vinegar, and 1 tablespoon lemon juice.
Cook and stir just till mixture boils and gelatin is dissolved.
Remove from heat; chill till partially set.
Fold in cucumber and celery; turn into 5-cup mold.
Chill till almost set.
Drain salmon, reserving liquid; add enough water to liquid to make 1/2 cup.
Remove bones and skin from salmon; flake.
Soften 1 envelope gelatin in 1/4 cup water; dissolve over hot water.
Combine mayonnaise, the reserved salmon liquid, and 1 teaspoon lemon juice.
Stir in salmon; blend in dissolved gelatin and spoon over cucumber layer.
Chill till firm, 3 to 4 hours.
Unmold; trim with lettuce.