Stir the tequila, mezcal, agave syrup, and bitters in a lowball glass. Add a large clear ice cube.
Express the orange peel onto the glass by squeezing it to release the oils. Or if desired, flame the orange peel (taking fire precautions as necessary!): Cut a piece of orange peel in a 2-inch circle. Hold the peel about 2 inches from the cocktail glass and warm it with a lit match. Then, quickly squeeze the peel to express the oils, causing it to flame briefly as the oil hits the glass.