stand-alone multiplexer only handles 4,800bps in and out, you will need one that handles 38,400bps for AIS unlike the multiplexer that is integrated into their AIS products.
Welcome to StandardHorizon.com
this is the SH page for the manuals
, the page for the GX2150 for Raymarine C & E series plotter is not going to help any as they are only concerned with the radio connection, take a look for the the connections for the GX2100 using the brookhouse mux, it shows there are spare inputs available for NMEA
for the pilot etc.