I think you might be talking about tiling terrains! You can do this using the TILE NORTH, SOUTH, EAST and WEST buttons in the drop-down list in the terrain editor. This allows you to create terrains which glue perfectly together at the edges... a continuation of the same fractal. This is too complex for me to explain without Bryce in front of me, so I suggest you look for a tutorial called "Tiling Terrains" or "Tiled Terrains" which clearly explains how to use this function... Good luck!