Identical printers, sub backend

[Fernando]

Hello.
I'm running Debian unstable's cups server v1.3.4. My problem is that I can't find a way to set up cups so it can differentiate two (or more) identical usb printers.
I've read somewhere that is possible to specify the printer serial number in the printer's URI in the form:
  DeviceURI usb://MANUF/Model?serial=55443355
but this didn't work at all.
I've also tryed to specify the device in the form:
 DeviceURI usb:/dev/usb/lp0
and this didn't work either.

The output of backend usb is:
direct usb://Xerox/Phaser%203124 "Xerox Phaser 3124" "Xerox Phaser 3124 USB #1" "MFG:Xerox;CMD:GDI;MDL:Phaser 3124;CLS:PRINTER;STATUS:BUSY;@"
direct usb://Xerox/Phaser%203124 "Xerox Phaser 3124" "Xerox Phaser 3124 USB #2" "MFG:Xerox;CMD:GDI;MDL:Phaser 3124;CLS:PRINTER;STATUS:BUSY;@"

Although this seems to differentiate these two printers (USB #1 and #2) the URI showed is the same. So my question is, what's the way to indicate CUPS which printer has to be used?

Thanks in advance,
Fernando