You can't plug
two NMEA-2000 devices directly into each other. As DeepFrz has already mentioned, you need terminators and power. I suggest geting a NMEA-2000 starter kit. These typically come with a multi-way connector block, 2 terminators and a 12V supply. You add two drop cables
and you are then also setup for future expansion should you want to add more devices later.
Alternatively, if you only ever plan to have the XB-8000 and MFD then can wire the two together directly via NMEA 0183
using ordinary two conductor wire.