Huxley presents us with two worlds for several reasons. Firstly either would could be seen as (an) utopia or a dystopia depending on your perspective. If you like order control, economy, and servitude for stability, then BNW is for you. If you love traditional values and emotions, then the reservation is your version of Utopia. Secondly, he needs two different worlds for the juxtaposition of the inhabitants and the values of each. The reader needs something to compare the other to just as the characters need something to compare the their world with. BNW would not be as effective if it were solely about BNW and there was nothing else.