Try using the ap option to associate with the BSSID of the desired wireless LAN.
Description and examples from the Wireless Tools (iwconfig) man page:
ap Force the card to register to the Access Point given by the
address, if it is possible. This address is the cell identity of
the Access Point, as reported by wireless scanning, which may be
different from its network MAC address. If the wireless link is
point to point, set the address of the other end of the link. If
the link is ad-hoc, set the cell identity of the ad-hoc network.
When the quality of the connection goes too low, the driver may
revert back to automatic mode (the card selects the best Access
Point in range).
You may also use off to re-enable automatic mode without chang-
ing the current Access Point, or you may use any or auto to
force the card to reassociate with the currently best Access
iwconfig eth0 ap 00:60:1D:01:23:45
iwconfig eth0 ap any
iwconfig eth0 ap off