Heat 2 Tbsp. dressing in large skillet on medium-high heat. Add mushrooms; cook 5 min. or until liquid is evaporated, stirring frequently. Spoon into large bowl; cool slightly. Meanwhile, partially cut each tenderloin lengthwise in half, being careful to not cut through to opposite side of each. Cover with plastic wrap or place in resealable plastic bag; pound to 1/2-inch thickness.