Hi. I have recently purchased a helix 9 g2n mega si and a helix 9 g3n mega si. I was told at time of purchase that using a network cable would allow me to use one navionics+ card for both my units. I have wired them up but am only able to see the contour mapping on the unit with the card installed. Was I told a lie or am I missing something. Any help would be appreciated.