I don't think you can do what you want to. AFAIK
, the reason is that the AIS port runs at 38.4K baud and the DSC port runs at 4800 (or maybe 9600?) baud.
You will need two serial ports, 1 at 38.4 to receive the AIS data from the SH radio and another at 4800 baud to send the GPS position data from the computer to the SH radio.
The serial ports can be real or virtual. If virtual, you will need two USB connections.