I want to join two different home networks together like so:

PC A1      PC A2                              PC B1      PC B2
  \         /                                   \         /
   Gateway A      <----- ethr. cable ----->      Gateway B
       |                                             |
  ADSL modem A                                  ADSL modem B

Both networks are of the basic residential type with identical configuration, with all PCs running Vista/7. The point is to temporarily join two apartments in a building for gaming and file sharing, and the holy grail would be:

1. PCs on network A can access PCs on network B and vice-versa (file shares and gaming).
2. Each network uses its own internet connection.
3. Data between networks shouldn't take a trip through the internet (broadband upload speeds are severely capped)
4. A network's internet access should continue working if the joining cable is disconnected with minimal configuration changes.

How closely can this be achieved?