You have the right idea with the volume. As you ease your way into training twice a day, you should be able to adapt and take on more strenuous training sessions. Although, if you push the heavy squatting twice in a day continuously, those adaptations will quickly turn into injuries if your not too careful.
The human body does adapt as Tamara said but the human body can only take so much. If we just continued to adapt and grow without ever breaking down then we would be all be snatching/C&J world records all the time.
Those European country's programs may show them going heavy twice a day but keep in mind that they have been training since they were 12 years old, all they do is train, eat and sleep, and there's a good chance they have a special substance that's helping them recover a lot faster.
Your commitment is admirable just remember to be smart about your periodization. Also make sure you are taking the extra steps for recovery (i.e. diet, sleep, ice baths etc).