Stir together first 6 ingredients. Chill, if desired.
Spread 3 tablespoons pimiento cheese evenly on 8 wheat bread slices. Layer with white bread slices, and spread each evenly with 3 tablespoons pimiento cheese. Top with remaining wheat bread slices.
Trim crusts from sandwiches using a serrated knife; cut each sandwich into 4 triangles.
Note: Freeze remaining 1 1/2 cups pimiento cheese up to 2 weeks, if desired. Thaw in refrigerator.