In a large glass jar with lid, layer orange slices and jalapenos. Top with liter of tequila, seal and refrigerate. Save original tequila bottle. After 1-2 days, remove only the jalapeno slices and discard. Leave orange slices in tequila for a total of 4 days. Strain tequila and return to original bottle. Chill.
To make the simple syrup combine water and 1/2 cup sugar in a microwave-safe glass measuring cup. Microwave at HIGH 2 1/2 minutes, stirring to dissolve sugar; cool.
For each margarita, add 3-4 lime wedges and a handful of fresh cilantro to a tall glass. Muddle until lime juice is released. To glass add 1 1/2-ounces tequila, 1 1/2-ounces Grand Marnier (or Cointreau), 1 1/2-ounces simple syrup, top with 1 1/2-ounces fresh lime juice. Add ice and shake well.
Dip rim of glass in coarse salt. Fill with margarita. Garnish with lime wedge.