I guess it depends as on how hard or long the individual sessions are. You certainly don't want to overtrain muscles so they don't recover and your progress stalls.
it doesn't take as much training to break down the muscle as people think if you are training the right way.
i've talked to mr O winners and competitors on my podcast and none of them train twice a day so not sure what benefit you would have as a gym rat doing it unless you work at the gym and are bored or something