I don’t think that this is possible for two reasons:
#1 is that it would take a lot of communication between the units to keep the map on the second unit updated. The only way I think that this could be done is with an Ethernet connection which only some of the newer units have. I am also not sure how much this would tie up the processors in both units…
#2 is that I doubt that Navionics would allow this. They are in the business of selling maps and mapping chips and so would not want to allow this. Not sure what legal rights they have but I bet it would get “messy” if we could do this.