After reading the documentation and Google's developer guide, I did not find a to add a second address line.
For example, if I want to store this address as a Google contact :
Mr. Ace Person
Amphitheatre Village Premium Outlets
1600 Amphitheatre Pkwy
Mountain View CA 94043
I can add the street as demonstrated in the developer guide :
StructuredPostalAddress postalAddress = new StructuredPostalAddress();
postalAddress.setStreet(new Street("1600 Amphitheatre Pkwy"));
I couldn't find a structured solution to add a "2 line street address".
Obviously I could add one of the two lines as a house-name (setHousename(housename)) or Neighborhood (setNeighborhood(neighborhood)) but this is not quiet right.
Unfortunately, there is no document or feature that support what you want to accomplish. According to the current document pertaining to the element's property which you can choose what appropriate property to work around you 2nd street line. I hope it helps.