Combine chicken pieces and shrimp in a large resealable plastic bag. Spicy Pineapple Marinade: Add pineapple juice, lime juice and jalapeno to bag. Refrigerate to marinate for 1 hour.
Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. Soak 12 bamboo skewers (if using). Remove chicken and shrimp from marinade to a bowl.
Pour marinade from bag into a small saucepan. Whisk in brown sugar, vinegar, cornstarch and 1/4 teaspoon each salt and pepper. Bring to a boil over medium-high heat. Boil 3 minutes. Remove from heat and divide in half.
Alternately thread 2 pieces of chicken, 2 shrimp, plus pineapple and green peppers onto each metal or presoaked bamboo skewer for a total of 12 skewers. Season with remaining 1/4 teaspoon each salt and pepper.
Grill kabobs 3 to 4 minutes. Turn over, continue to cook an additional 3 to 4 minutes, brushing with half the spicy pineapple sauce, until chicken and shrimp are cooked through. Serve kabobs with reserved sauce on the side.