YouTube user k2nccvids has posted a video showing him decoding and plotting both AIS channels simultaneously on a Windows PC with free software. To do this he uses SDR Console and two AISMon instances. SDR Console is a general purpose SDR GUI that supports the RTL-SDR. One of it’s major advantages over other SDR software is that it can tune to multiple signals in the same swath of tuned bandwidth simultaneously and output their audio to different virtual audio cables.
k2nccvids used two AISMon instances, each one connected to a separate virtual audio cable outputting AIS audio from SDR Console. He set AISMon to output decoded UDP packets on two different ports. Then he created two UDP listeners in OpenCPN for plotting, one for each port.