Its pretty damn hard to do though. When I say master I mean truly master though. I'm going to use musicians as an example of mastery because I can communicate that better than speculating on mastery of skiers.
John Petrucci.
Incredibly talented guitar player, and he has undoubtedly mastered his instrument (sure, thats subjective but lets just go with that for now).
However, this came at a cost - 6 hours a day when he was attending Berkely for guitar. Pretty crazy when you think about it. Classes all day on guitar, practising in all his spare time.
Now, the generally agreed upon time requirement for mastery is 10 000 hours of dedicated, focused practise. At 6 hours a day, not missing a single day, this would take 4 and a half YEARS. 6 hours a day for most people is completely unfeasible. 3 hours a day would take 9 years. Its difficult to even practise 3 hours a day for most people though, because doing good, focused practise is incredibly exhausting. Lets say an hour a day. That means it would take 27 years (I need to practise more...).
Now sure, you can argue that if you master one instrument you can pick up another and master it, and that may be possible but you can't do your 6 hour guitar practise and your 6 hour piano practise in one day and expect to not go insane or broke. If you try to master something else, you WILL lose your mastery because there's not enough time to practise.
Another phenomenon that prevents you from mastering more than one thing is that its very difficult to build skills that you haven't already been partially trained for later in life. If you achieve a mastery by the age of 25, you are completely unfit to master something else because you are far past your prime learning window. This contributes to the difficulty of achieving multiple masteries.
With these phenomenon taken into account, I personally believe that its not feasible to master 2 things within a lifetime, never mind maintaining that mastery. However, thats just my opinion and I'm completely open to debate! For further reading, check out the book "The Talent Code". I really enjoyed reading it, and if you ever want a good kick in the face to get you started on your way to your dreams, its definitely worth your time.
Sorry for going off topic.