In a large bowl, dissolve orange gelatin in 2-1/2 cups boiling water. In another bowl, dissolve grape gelatin in remaining boiling water; set both aside for 30 minutes.
In a large bowl, whisk milk and pudding mixes until smooth, about 1 minute. Quickly pour half of the pudding into each bowl of gelatin; whisk until well blended. Pour into two 13-in. x 9-in. dishes coated with cooking spray. Chill for 3 hours or until set. Cut with 2-in. Halloween cookie cutters.