How to Make It
Place chopped bacon in a small nonstick skillet over medium; cook, stirring occasionally, until crisp, about 6 minutes. Drain on paper towels.
Stir together bacon, mayonnaise, chives, zest, and 1/4 teaspoon of the salt in a small bowl. Spread mayonnaise mixture on 10 bread slices. Layer 5 of the mayonnaise-coated bread slices with red tomato slices; sprinkle with 1/8 teaspoon each of the salt and pepper. Top with remaining 5 mayonnaise-coated bread slices, coated side up; layer with yellow tomato slices, and sprinkle with remaining 1/8 teaspoon each salt and pepper. Top with remaining 5 bread slices.
Cut each sandwich into 4 pieces, and serve immediately.
Prepare the sandwiches 1 hour ahead, and then refrigerate. Transport in a container lined with paper towels to keep them in place.