People are underestimating the effort of riding 2 consecutive GTs and going for GC in both. For the double Tour-Vuelta just check Evans 2007 (dropped from 2nd to 4th in the last two decisive stages) or Sastre 2008, who was never a threat to you know who. I don't think Froome can compare to Sastre regarding his capabilities to podium in two consecutive GTs.