Choose which Airport to connect to
What you could do, is give them the same name, but dial down the transmit power for each.

The current AirPort Utility (the 6.x version) doesn't support this, but the old 5.6.x version can still be downloaded here:
http://coreyjmahler.com/2013/10/24/airport-utility-5-6-1-on-os-x-10-9-mavericks/