You could say that Venice represents the "real" world and that Belmont represents an ideal, fantasy world. In Venice, the talk is initially about money, economics, and business. Bassanio needs money; he goes to Antonio. Antonio needs money; he goes to Shylock. The supposed order of this "real world" Venice is not so orderly or modern (for its time) once we see how much greed guides and disrupts the city. In Venice, there is a reciprocal antagonism between Christians and Jews.
This world of money and greed is balanced by its opposite: Belmont. In Belmont, we have a fairy-tale type of challenge for Portia's potential suitors. In Belmont, the subject deals more with love than money and greed. This is where Antonio travels to and eventually wins Portia by choosing the lead casket. Jessica and Lorenzo flee to Belmont in order to escape Shylock's wrath. And it takes someone from this magical place (Portia) to come to Venice in order to save Antonio.
So, the tension is provided and supported by this back-and-forth between the two places: greed and corruption are represented in the real world (Venice), kindness and love in the ideal world (Belmont). This simplification makes Venice sound like the "bad place" and Belmont is the world where dreams come true. However, Belmont is idealistic to a fault; Portia and Nerissa spend a lot of time talking about the suitors' challenge, idle talk compared with Antonio's plight. Portia is rich; she doesn't really have to deal with the kind of economic problems that Antonio, Bassanio, and Shylock do. Also, those in Belmont seemingly don't have to deal with the religious tensions that those in Venice do.
This simplification also ignores the complications of Shylock himself, who some critics see as villain, while others view him as a victim of anti-antisemitism. However, the simplification of place is useful to show the tension between the two cities. Consider the tension from Bassanio's perspective. In Venice, he is struggling financially and must resort to borrowing from Antonio, who then must borrow from Shylock: complications ensue. In Belmont, he must choose the right casket (surely an anxious decision), but his situation is not dictated by money and greed; it is a fable dictated by love. It is a fairy tale when compared with his financial struggles in Venice.
Another way to look at the two cities is by gender. Venice seems to be run by men and laws whereas Belmont is run by women and fantasy. This generally is the case but this is also a simplification. The simplification is undermined when Portia, from the fantastic Belmont, comes to Venice and uses its own laws to save the day. Thus, the princess from the fantasy land successfully adorns the clothes (literally) of the the man's world in Venice and argues the law better than he could. In this way, Portia transcends the real/fantasy, man/woman divisions of the play in terms of the themes of place and of gender stereotypes.