Search the forum for “World Browser”. There’s a nice tutorial video and thread about it. It’s an experimental feature, but it allows you to connect up maps in a grid for streaming, along with origin rebasing (i.e., so you avoid floating point precision problems). It can import tiled heightmap terrain from World Machine.
Right now it doesn’t work with multiplayer though, but you could use regular streaming to do a smaller scale prototype and replace it with the World Browser later, if you’re doing multiplayer.