Do I need to buy the same steam game twice for two different operating systems?

If it becomes available for Linux, then you won't have to purchase it a second time; there is no guarantee that it will come to Linux, though, which means you'll only be ably to play it on Windows (or possibly with WINE).

Steam Play is how valve refers to cross platform games. It looks like every cross platform game that is available is a Steam Play game, which means that if you buy it on one platform, you can play the game on any platform that it's available for. But as I said, the game needs to be released for Linux for that to happen.