The reason there are two passages through what seem to be limbo is that Inception is Christian allegory. If you look at the structure of the film you can see No

The reason there are two passages through what seem to be limbo is that Inception is Christian allegory. If you look at the structure of the film you can see Nolan using the opening and closing heist sequences as allegorical bookends to demonstrate Cobb's character development. In the first Cobb is a faithless and money-oriented thief who embraces violence and selfishly abandons his team when this fails ("every man for himself"). At the end Cobb takes a "leap of faith" when he rejects Mal, whose seduction of him is framed as a temptation of faith ("you don't believe in one reality anymore.... so choose to be here"). Cobb reaffirms his faith in his children "up there", rejects violence even when he is attacked and then sacrifices his own life to save Saito. So you're not supposed to worry too much about the logic of the dream levels, since all dreams are basically metaphors for life: mazes where people "get lost" and from which they need to "die to wake up". The only thing that makes limbo special is that it is particularly symbolic. Nolan is presenting a metaphor of life itself as a Penrose staircase, and portraying faith as the way out. When Ariadne shatters the mirrors that trap Cobb in a recursive chain, the image is symbolic: she is a gift from Cobb's father ("ask and ye shall receive") and her role in the film is to guide him out of the maze that is the mortal world. This is presumably why she is the character who accompanies him to immigration. For more evidence that this is intentional, look at the overwhelming creation imagery and the narrative emphasis on father-son alienation and reconciliation (with Fischer as with Cobb). Look at the curious way Michael Caine seems to be playing God when he shows up in Paris. And then look closely at the ending, which shows us neither a dream nor reality. What Nolan presents is symbolic: we see Cobb's judgment and forgiveness of sins at immigration, and then his reunion with his family in the heavenly garden. The film closes with Cobb ignoring his totem (as a crutch of faithlessness it is no longer needed) and then his son James (who represents faith and like his sister shares an apostolic name) telling him that they are building a castle on a cliff. A what? That last bit circles back to the opening shot of the children on the beach. It is a bookend reference to Matthew 7.24 and the parable of the wise and foolish builders. The contrast (beach -> cliff) reinforces Cobb's character journey while telling us that the ending is NOT a dream (something reinforced by the lack of the water imagery associated with the other dream levels). It also reinforces the parallels Inception creates between the buildings of limbo and the sandcastles on the beach, and explains why all are ultimately washed away by water just as death washes away life in the Christian parable. Brilliant movie.