Cut a slit into a wedge of lime, then run the lime around the rim of two cocktail glasses. Dip each cocktail glass into a shallow plate filled with salt (and optional lime zest for extra flavor).
Pour the orange liqueur, rum, simple syrup, lime and orange juice into a cocktail shaker half-filled with ice.
Shake well to combine.
Strain into your salt-rimmed cocktail glasses with crushed ice, and enjoy!