I believe he means he’s using two images (pieces) tiled together to form one large map.
The way to do this is define the zone separately in each section. Give them the same name, and no one will be able to tell the difference.
With one major exception: It is possible to have stacks ‘snap to’ the same place, but be different stacks, if they are on different boards (or just overlap more regularly). If you are breaking a hexgrid map into pieces, it is strongly advised that you divide it near the hex boundaries.
Also, Vassal 3.2 handles large images quite well, so if your image software can handle it, I suggest keeping the board as all one image, and everything else will be simplified.