Actually from a practical point of view, and looking at what is happening on Flightradar24.com, you will see that if a plane is over an ocean going west it will be given an even FL, either 360, 380 or 400. And if it is going east then the usual FLs are 370, 390 or 410.
The exact track most likely takes into account the jet-stream winds aloft, so the east goers are separated from the west goers, by quite a distance.