[GPSNavX]

The VHF should combine the NMEA data and output it on one connection eliminating the need for the USB GPS. So you should be able to use one Keyspan. Unfortunately I have had no luck getting on the SH website to confirm this. At the very least you would get the AIS data from the VHF and continue to use the USB GPS.

Here is the connection flow:

SH VHF->AIS/GPS NMEA Data->Data Cable->Keyspan->Mac->KeySerial1->GPSNavX