Stir together the 1 tablespoon olive oil, lemon juice, chives, basil, mint, and garlic in a mixing bowl. Add shrimp, crab, tomatoes, and onion; toss to coat.
Cut the bread into 48 thin slices; brush one side of each slice with some of the 2 tablespoons olive oil; sprinkle lightly with pepper.
Arrange bread slices, brushed side up, on a baking sheet. Broil 3 to 4 inches from heat for 1 to 2 minutes or until toasted. Arrange on a serving platter with oiled sides up. If desired, spoon seafood mixture on each slice. Garnish with fresh basil or chives, if desired. Serve the appetizers at once. Makes 48 appetizers.