Process 1 1/2 cups frozen raspberries, 1/4 cup orange juice, and 1/4 cup water in blender until smooth. Using a spatula, press mixture through a sieve over a bowl; discard seeds. Rinse blender, and pour the berry mixture back into it.
Add 2 tablespoons sugar, 1 tablespoon fresh lime juice, 1 ounce tequila, and 4 ice cubes; process until smooth. (Simply omit tequila for a nonalcoholic version.)
Transfer to a bowl, cover, and refrigerate.
Rinse the blender, and add 1 tablespoon water, 2 tablespoons sugar, 1 tablespoon fresh lime juice, 1 ounce tequila, 1 1/2 cups sliced peeled peaches (frozen are OK), 1/4 cup peach nectar, and 3 ice cubes; process until smooth.
Pour the mixture into 4 tall glasses. Gently pour the raspberry mixture onto the peach mixture to create a second layer.
Garnish with additional berries and peach slices, if desired.
