You do not need to create any USB serial connection in opencpn. Opencpn should have just a network connection: localhost 10110 TCP (the SK connection).
Then you should connect both serial NMEA 0183 devices to signal k in serial app - connections tab. Data from both devices will de available in TCP localhost 10110 multiplexed.
Then you should connect both serial NMEA 0183 devices to signal k in serial app - connections tab. Data from both devices will de available in TCP localhost 10110 multiplexed.