In a medium mixing bowl stir together the softened cream cheese and mayo until combined.
Add the Cheddar and Monterey Jack cheeses, Worcestershire, garlic powder, onion powder, and cayenne. Mix and season to taste with salt and pepper.
Add the pimentos and green onion and fold in until combined. Use the pimento cheese spread immediately or add to a resealable container and refrigerate until ready to use, about 1 week.
Arrange 2 of the slices of bread on a cutting board. Scoop ⅓ of the pimento cheese mixture onto one of the slices of bread and spread evenly over the bread to the edges.
Close the sandwich and cut in half diagonally. Repeat with the remaining bread and pimento cheese mixture.
Notes
To prep the sandwiches in advance, make the pimento cheese in advance then store refrigerated in an airtight container until ready to assemble and serve the sandwiches, up to a week in advance.