1/2 cup fresh lime juice (from about 5 limes), plus wedges for garnish
1/2 cup fresh orange juice (from 1 orange)
1/3 cup sugar
Two 12-ounce bottles Mexican pilsner, well chilled
Stir together the tequila, lime juice, orange juice and sugar until the sugar dissolves. Pour the mixture into a baking dish and freeze until hard, about 5 hours. Scrape the mixture with a fork to break it into small ice crystals.
Spread 2 tablespoons salt on a small plate. Moisten the rims of 4 large martini glasses with a lime wedge and invert onto the salt. Distribute the frozen margarita among the glasses, and then top each with half a beer. Garnish with a lime wedge and serve.