So when would I recommend not doing two workouts a day? How would I figure out if this is right for you ? First of all if you are new to exercise or have been sedentary for quite a while and are just getting back into exercising regularly, I would not recommend two workouts just yet. You and your body need to get used to working out regularly. I usually recommend three times per week initially with one 40 minute workout each day.
If you are not new to exercise and have been doing it for a while and you think you would like to try two workouts a day just make sure you pay attention to your body and what it needs. If you feel excessively tired, almost exhausted after doing both workouts or you feel dizzy and light headed during either one of the workouts, you need to reassess and maybe think about going back to once per day for a bit or work your way up. Try doing two workouts in a day just once per week and keep the rest of the week the same and build up from there.