Melt semi-sweet chocolate and peanut butter in medium microwaveable bowl on high 1 1/2 to 2 minutes or until chocolate is melted when stirred, stirring every 30 seconds. Stir until blended.
Pour over grahams; spread with spatula to cover completely.
Repeat microwave melting steps with white chocolate.
Drop spoonfulls of the white chocolate over chocolate-covered grahams. Immediately cut through the chocolate mixtures with knife several times for tiger-stripe effect. (I used a toothpick)