Time is usually of the essence for people. In most cases, workouts are based around one’s day. In rare cases (people with the time to do so), the day is based around one’s workout. With that being said, some people have the time to work out in the morning and the evening. These “two-a-days” are uncommon for many, but for some they are a reality.
When it comes to two-a-day workouts, some people attempt to lift twice a day. The downside to this approach is the body not having enough recovery time. If the body is not recovering properly, progress can be halted and even reversed, which could lead into overtraining. Overtraining has a multitude of issues beside stalling progress.
Morning is an awesome time to get cardio in, especially cardio on an empty stomach. By doing some low moderate intensity cardio on an empty stomach, you are burning primarily fat.
Cardio is great of course for fat loss, but also is good for (you guessed it) your cardiovascular system. Morning cardio will also help set the pace for your eating for the day by keeping your metabolism in check. Finally, by getting cardio done in the morning, it leaves more time for you to lift weights in the evening.
Looking at lifting weights in the evening, there are some cognitive (mental) advantages. When you work out in the evening, the body is awake and rolling. The brain has been talking to the body the whole day, so communication is better established. What does this mean for you? It means you will feel a little more encouraged and mentally focused to lift. This of course means more intense and effective workouts. And remember, no need for cardio because you already did it this morning. So you can be more intent with the content of your lifting.
So, if you find yourself with the time and ability to get a two-a-day workout in, keep the cardio for the morning and the lifting for the evening. This seems to be the best utilization of time and physical capabilities, because you are taking what the body is giving you for that day and using it for your gain.