Preheat grill to high. Place mango and pineapple on grill, cook for 2 minutes, turn over and cook for 1 minute more. Remove from grill to a cutting board and let cool. Roughly chop the fruit and put in large glass or cocktail shaker with 1/2 cup of ice, muddle, and add simple syrup, lime juice and tequila. Shake or stir to combine.
Combine chili powder and kosher salt on a small plate. Rub the rims of 2 serving glasses with pineapple wedge, and coat the rims with the chili powder/salt. Fill each glass 3/4 full with ice and pour in cocktail mixture.