it's no big deal to do twice a day, it's only overtraining you if you're not getting enough rest to account for the workload you're putting on your body.
Overtraining doesn't happen in a workout or even a week, it takes a period of time and if you get enough rest and switch up your routine you'll be fine.
Double split training is used by bodybuilders at all levels and is often helpful in generating intensity for both body parts. If you find yourself getting too burned out after a back workout and unable to hit bis with full intensity, go back and hit them hours later with full intensity.
Bottom line is, if you want to switch things up, double split is fine, just get enough rest and plenty of nutrients, expecially that all-important
protein.