Fill a cocktail shaker with cubed ice (about 1 cup). Add 2 oz white rum, 0.5 oz dark rum, 0.5 oz Cointreau, 0.75 oz fresh lime juice, 1 oz fresh grapefruit juice, 0.5 oz simple syrup, and 2 dashes Angostura bitters into the shaker.

Shake

Secure the shaker and shake vigorously for 10–15 seconds until the outside of the shaker feels very cold and the mixture is well aerated and slightly diluted (about 15–20% dilution).

Prepare glass

If using crushed ice for presentation, fill a highball or Collins glass halfway with crushed ice; otherwise, add a few large ice cubes to the glass.

Strain

Double-strain the shaken cocktail into the prepared glass over the ice using a Hawthorne strainer and a fine mesh strainer to remove small ice shards and pulp.

Finish and Garnish

Top the strained cocktail with 2 oz chilled club soda. Gently stir once with a bar spoon to integrate the soda without losing too much carbonation.

Express the oils from a grapefruit twist over the drink by holding the twist over the glass and giving it a quick fold, then drop the twist in (or perch a grapefruit wheel on the rim).

Serve immediately with a short straw. Enjoy responsibly.