udev: attributes rules to identical devices

Hello everybody, 

I am working with Kubuntu 13.10. 
I need to assign permanent names to some modems that I am working with. 
This would be possible by creating a rule in /etc/udev/rules.d/ and defining 
a symlink, something like this:

KERNEL="ttyUSB[0-9]*", ATTRS{serial}="0000:00:1d.0", SYMLINK="Modem0"

The problem is that all these modems have the same serial, so I cannot 
distinguish them and cannot assign a different symlink to each one. 

If it can help, the output of lsusb with two modems connected is this:
 
Bus 006 Device 002: ID 0403:6001 Future Technology Devices International, Ltd 
FT232 USB-Serial (UART) IC
Bus 005 Device 002: ID 0403:6001 Future Technology Devices International, Ltd 
FT232 USB-Serial (UART) IC

Is there a way to do this? 
Thanks.

Theo