Halve 2 pounds prickly pears (about 8) lengthwise, scoop out the flesh and chop. Mix with 1/2 cup sugar in a saucepan; let sit until juicy, 5 minutes. Bring to a boil, then remove from the heat. Mash the fruit and let sit 10 minutes. Strain through a fine-mesh sieve and refrigerate the syrup until cool, 1 hour. Combine the cooled syrup, 1 1/2 cups tequila, 1 cup orange juice and 1/2 cup lime juice in a pitcher. Add 3 cups cold seltzer. Serve over ice.