from ftdi tech support, FYI
I suspect both your device have the same serial number. This can be confirmed by plugging in one device at a time and using USBVIEW to view the serial number.
To change the serial number of one device you must uninstall the current driver using Control Panel > Add/Remove Programs.
Install the D2xx driver:
Install MPROG and use the default template provided.
Uninstall the D2xx driver using Control Panel > Add/Remove Programs.
Re-install VCP driver for both devices and 2 distinct COM ports should be allocated.