Combine rhubarb, strawberries and sugar in small saucepan set over medium-high heat. Cook, stirring occasionally, 8 to 10 min., or until sugar is melted and fruit is tender. To cool quickly, spread mixture in thin layer on shallow dish or plate. Refrigerate 10 min. or until completely cool.
Transfer fruit mixture to blender; pulse until smooth. Add ice, tequila, lime juice, orange liqueur and salt. Pulse until ice is slushy.
Divide mixture into 4 margarita glasses. Garnish rims with lime rounds.