-First of all, I'd like to say, Im only typing with on hand so, there are bound to be alot of mistakes -
As I sent in a Pm to you, Sami, its pretty easy to do cities with with 1-2 airports, If the second one is very small.
Examples: for Seattle you have SEA and BFI, the main is Seattle and a very small amount pax go through BFI [Less than 20,000].
Another: for Portland you have three, PDX, Troutdale, and then Hillsboro. The last two are like BFI, with very few passengers. To get more accurate demand, these airports can - A: Be left alone and have indiviual route data submitted, B: Demand is generated by only the local communities. The main airports obviously have demand from their whole cities population.
For Airports like DFW/DAL and IAH/HOU, where we have a clear main airport.... but there is still a secondary airport that has noticable service. So, as before the major airport has demand from the whole area, but this time this larger secondary airport takes demand from a larger chunk of the local area. In both cases, HOu/DAL are located closer to the city's center. So they might take demand from the county/cities/region/prefrecture/provence/etc that they are in.
Alright, Im going to split this post into two as.... The forum does funky thing after making it longer than this, which makes it hard to type.