Let me tell you this is absolutely madness! Let me also remind you that I'm a firm believer of fitness being a lifelong marathon and not a sprint. Just because you workout twice as much in a given period of time you'll end up getting stronger and fitter? No freaking way. In the long run, it'll harm you and could even keep you out of the gym forever (yeah, slight drama here but there's certain truth to this) if you sustain a serious injury! You need to have enough rest to recover.
I'm a big fan of Lotus (the sports car ya (used to own the Elise Mk1) and not the Indian restaurant) and I subscribe to their philosophy "LESS IS MORE".
Fitness is not all about working out at the gym. It's also about what you do outside of the gym such as your nutritional intake/diet, rest etc. What you do outside of the gym constitutes about 60-70% of how you look.
So how do you avoid overtraining?
1) Have the right training frequency- Train on alternate days. Training more then 2 days in a row is very difficult for your body to recover from. When you train on alternate days, you allow more recovery time and thus enable your body to get stronger.
2) Take a training break- Completely stop training every 12 weeks for 1 week. This will provide ample time for your body to recover and your joints to heal.
3) Do High Intensity Interval Training ("HIIT")- Don't waste your time doing 45-60 minutes of low to moderate impact cardio. Do HIIT instead. With HIIT, your workout gets harder, more challenging and more fun resulting in more fat/calories burned in less time.
So remember..."LESS IS MORE" and you have all to gain!