Place the steak, chicken, onion, peppers, fajita seasoning, oil and water in a large zip top bag and seal. Using your hands, mash around the ingredients in the bag to mix well. Allow to marinate for 15 minutes or up to 24 hours.
Cook the steak, chicken, onion and peppers on the grill over high heat, approximately 5-7 minutes. Turn the steak and chicken over, continue cooking until done. Approximately 2-3 minutes. Transfer the steak, chicken, onion and peppers to a platter and cover to keep warm. Place the cedar planks on the grill and cook for 3 minutes. While the planks are warming, slice the steak and chicken into thin strips across the grain.
Turn the planks over and top with tortilla chips. Arrange the steak, chicken, onions and peppers on top of the chips. Place the VELVEETA® evenly over the nachos and close the grill. Turn the heat to low and cook until the VELVEETA® has melted.
Remove from the grill and serve immediately. Top with cilantro, jalapeno, tomatoes and sour cream if desired.