We would not recommend trying to do HIIT more than once per day for an average individual. The purpose of a HIIT workout is to give 100% effort during the workout and then allow proper time for recovery (ranging from 24-48 hours depending on your fitness level). A second HIIT workout would not be as efficient or effective as your first workout and can lead to injury if you are over-exhausting yourself. Instead of two HIIT workouts, you can add power walking or low-intensity cardio later in the day for active recovery. You don’t need to constantly give maximal effort to get the benefits of HIIT.