You can't compare 2011 with 2015.
Unlike then Contador collected experience from doing the giro-tour double. He also has now more help from trainers. I don't know if that will help but most of them come from sky so i can imagine it'll
Giro 2011 was way harder than 2015 and the competition was way stronger assuming no one else of the big 4 will participate. Menchov, nibali, scarponi, purito, kreuziger. At that time those were legit contenders, for sure scarponi who was in his prime so Contador was forced to be 100%. I think he can be a bit less and with them sky guys it should work out.
His performance in the tour 2011 was also impressive. Without bad luck he could have won that race. Of course competition is stronger now but that's also an advantage for Contador. In the tour 2011 all of them looked at Contador, all the time. Now you've nibali, froome and quintana. This will make it easier for Contador.
In conclusion, he can do the double, with some luck
