Combine lime zest and juice, lemon zest and juice, sugar, and salt in large liquid measuring cup; cover with plastic wrap and refrigerate until flavors meld, 4 to 24 hours.
2. Divide 1 cup crushed ice among 4 or 6 margarita or double old-fashioned glasses.
Strain juice mixture into 1-quart pitcher or cocktail shaker.
Add tequila, Triple Sec, and remaining crushed ice; stir or shake until thoroughly combined and chilled, 20 to 60 seconds.
Strain into ice-filled glasses and serve immediately.
The longer the zest and juice mixture is allowed to steep, the more developed the citrus flavors in the finished margaritas.
We recommend steeping for the full 24 hours, although the margaritas will still be great if the mixture is steeped only for the minimum 4 hours.
If you're in a rush and want to serve margaritas immediately, omit the zest and skip the steeping process altogether.