I doubt Contador was clean in 2008 when he won the Giro and the Vuelta. But that aside, your point remains that it is possible to peak for the first and third grand tours of the year.
But Nibali's camp has acknowledged that he isn't as in-form right now (estimated 15-20 max watts lower than his Giro form).
I think the point remains that it is very difficult (or unlikely) to have two maximum peaks in a given calendar year.
However, I may be wrong in thinking that Horner is barnstorming this year's Vuelta because everyone else's form is a bit below max peak. Apparently his ascent on yesterday's stage was the fastest-ever ascent of that climb, and his power numbers are off the charts. However, remember that VAM is a fairly rudimentary measure of true power and doesn't control for a number of other relevant factors.
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"c" is not a unit that measures tire width