Yes, if you double the amount of meat in a slow cooker recipe, the cooking time will typically need to be increased by about 25%, and maybe up to 50%. When you double the meat, you increase the volume of food in the slow cooker, which means it will take longer to cook all of the ingredients thoroughly. The exact increase in cooking time can vary depending on the type of meat, the size of the slow cooker, and the specific recipe you're using. To check if the meat is done, just use an instant read meat thermometer so you don't have to play the guessing game. A chuck roast needs to hit between 200˚F and 210˚F before it is ready to shred. Keep checking for tenderness and that the meat easily pulls apart.