If you're at all interested in training twice a day, you HAVE to start slow. Like I said before, if you jump right into a double session week, you'll burn out in a heartbeat (or lack of one, because you'll be so dead.) Take it one step at a time. If your workouts are geared toward general fitness, and include both cardio and lifting weights, I'd suggest doing light cardio in the morning (I'd say 10-20 minutes, but your definition of light may differ), then have a good span of time between that and your evening workout, which should also be relatively light (in terms of weight this time.) Try this one day a week, preferably a day you have free so that you can rest and recover between workouts. And when I say rest, I MEAN REST. Don't plan your family hike for that afternoon, just lay out in the sun, or go for a swim, or watch some TV, or take a nice long nap. Recovery is just as important as the workout itself. If you do it right, you should be ready to hit the gym again that evening.
If you're all cardio, it's a pretty simple setup to get two sessions into one day. Start with a light jog in the morning, then later that evening hit a serious cardio session, like your timed run, biking trip, swim, etc. Cardio is easy to overdo, but if you're serious about it your body should adapt quickly. Keep carbohydrate levels very high, and make sure to take care of those joints (especially runners), because they can get very sore very quickly. Stretch, ice, heat, foam roll, massage, do what you have to do to keep pounding the pavement. Many runners go for double sessions, so finding one to talk to about it isn't too hard a task. Look around, see what works for others, and try to incorporate their techniques into your workout.
For a bodybuilder, a double gym session is a GREAT way to increase volume on certain muscle groups. If it's back and bicep day, you can hit one group of exercises in the morning for a moderate number of reps, then really finish those muscles off in the evening with a completely different routine, going all out. Increasing workout volume is a fantastic way to gain size quickly, and I bet you'll be surprised at the results. Don't be afraid to hit the same muscles twice in a day, they can take it as long as you supplement them properly. If you're wondering exactly how to do this, I found a great post in the bodybuilding.com forums by username "labradarep" that explains the concept pretty well:
"Do the same bodypart(s) twice in a day. First session should be moderate number of sets in the 4-6 rep range. 2-3 exercises per bodypart(30-40 minutes max with 2 -3 minute rest periods). Rest 4-6 hours & then perform different exercises for the same bodyparts. 2-3 sets of 10-15 reps. 1 -2 exercises per bodypart (20 minute workout with very short rest periods)."
That in my opinion is the perfect way to ensure you don't overtrain, but still hit each muscle group for an AWESOME amount of volume. Try it out, and watch all the other guys in the gym start to pale in comparison to your massive size and muscularity.
If you're a strength athelete, like me, and want to incorporate some morning and evening material into your workout, I'd suggest squats in the morning, and your full, organized workout in the afternoon (my "organized" workout involves the Olympic lifts themselves, just in case you're wondering what that means. If you're a power lifter, it may include deadlifts, or bench press. Strongman may include things like axle presses, farmer's walk, deadlifts, etc.) Keep your body guessing, and do a workout that won't impact your body enormously for the day in the morning. When I do my set of squats in the morning, I squat heavy, but for low reps, and with large rest periods in between sets, just to make sure I don't burn out or expend too much glycogen too early on in the day. Keep it casual at first, go slow, and like I said, try one day at a time first.
When you start to adapt, obviously you can increase the frequency of these double sessions. Add another one in three or four days later, basically in an attempt to split the week up as evenly as possible. You don't want two days of double sessions in a row, at least not initially. Don't increase the workout intensity just yet. Personally I find that increasing frequency works much better than increasing intensity. I do 3-4 double sessions a week, and NONE of the morning sessions are EVER as intense as my afternoon ones, purposely so. Stay relaxed, focus on moving the weight, and keep yourself under wraps. Remember, you've got another workout later where you can go all out.
Now, I forgot to mention one thing, and it's in regard to your diet. Double days make you HUNGRY. And if you decide to skip out on meals, or keep your calorie level the same thinking your body doesn't need the extra food, you're ABSOLUTELY NUTS. YOU NEED TO EAT!!!!! Plan on increasing caloric intake from between 300-500 calories on double days (and possibly the day after), and when you start adding more and more double sessions, you have to consistently keep your body fueled. If you don't, you'll feel it, IMMEDIATELY. You'll feel like a zombie amongst the living. You'll be sore, unable to focus, drained, and barely able to move weights you practically tossed up before. Eat...more...food. I've found increasing carbohydrate intake works wonders. Have a sweet potato and a glass of milk, or a bowl of oatmeal and a whey and banana shake. No matter if you're a cardio-aholic, bodybuilder, or strength athlete, keep your body fueled, and you'll start to see the double sessions work wonders for your body.
If at any point in the week you begin to burn out, BACK OFF THE DOUBLE SESSIONS. Seriously, eat some food, get some good sleep, and rest until you feel like you can hit the gym that hard again. Double days take a lot out of you, so don't be surprised if the adjustment process is long. Don't rush it.
Basically, to be successful with double sessions, it all boils down to two things: Eat more, and sleep more. Keep your diet clean and VERY full, and your bedtime early and consistent, and you'll find hitting the gym twice a day isn't so bad. I love it, and if I could set up a cot in the corner, I'd probably be in the gym all day. So to all my fellow gym lovers, rejoice, if you want to spend even more time pumping the iron or sprinting to nowhere, you're more than welcome to. In fact, it can do wonders for your strength and physique. You'll get leaner, stronger, faster, bigger, and more powerful faster than you ever thought possible. So what are you waiting for? The gym beckons.
Lift hard, run fast, eat big, sleep deeply, and as always, GOOD LUCK!