There are a number of ways to open a port of the FT2232C using information returned from FT_ListDevices.
FT_ListDevices can be used to return a device description or serial number of the device. FT_OpenEx can then be used to open by description or by serial number. When FT_ListDevices is used to get information on a FT2232C device, the string returned is appended with a letter to indicate which port has responded (e.g. "USB <-> Dual Serial Converter A" or "FT012345A" for port A of the device). This string can then be modified by removing the last character and appending the letter corresponding to the port you want to open.
For example, using FT_ListDevices to return information on device index 0 may return a description of "USB <-> Dual Serial Converter B". Removing the last character of this string leaves "USB <-> Dual Serial Converter ". It is then possible to append either "A" or "B" to the end of this string which can then be used with FT_OpenEx to open the required port.
Ports may also be opened by index, but this cannot guarantee which port will be opened.