Am I missing the point here? Surely there are now 2 doors, and there is no more likelihood that the car is behind one or the other?
If the premise was simply "one of these doors has a car behind it, which one?" then you'd be right. But that's not what's been set up.
You're presented with two doors. However, the odds of winning have been stacked by previous events; ie, the two doors are not equal.
At the start of the game, you pick a door and have a 33% (rounded) chance of being a winner, yes? Think about that. You can now do what you like with the other doors, add more, remove one, paint them pink, set them on fire, write "here be goats" on them; it does not, can not change the likelyhood that your door contains a car or a goat - it will always be 33%, that was the choice you originally made.
In the second round, you're given another option. You can keep your existing choice (33% chance of being right, remember) or you can trade that choice for another door which, now that we've removed an obvious wrong choice, represents all other odds other than the one you already hold. 100% - 33% = 67%.