The above paragraph is really two topics, 2X XP and taming, but I'll just discuss the XP and leveling.
Any game is hard to "balance" for all the different modes of play. For instance it is pretty irrelevant how fast anyone levels when you are playing PVE. Certainly you want a challenge, but how fast you level is just one of hundreds of things that make the game challenging.
Now for PVP, if you are trying to reduce the speed that members of an Alpha tribe level, you pretty much can't win that game without make the single player experience hell. The alpha tribe has all kinds of "tricks" at their disposal that they use to level players. Can't say I know them all, but like said here, in 12 hours, the alpha tribe player might level up as much as 4x what a solo player can achieve. So really 1X or 2X, it doesn't really affect them, it only punishes the solo player.
I've used a few of these tricks myself. All I need is to join a tribe of even 1 player that has all the standard dinos. I can then use them to gather the resources needed to make all the items that give the most XP and level to 50 in a few days. If they have storage bins already filled with the resources and don't mind me using them all up, I can level even quicker. This was on an unofficial that I did this, I think with 4X for the XP rate, but doesn't really matter the rate, with the right techniques that are really only available in a tribe (the bigger the better), you can level much quicker.