Hi Jason,
In this case, provided that you line up the landscape objects properly, I think you can eliminate those seams by placing the landscapes in a Connect Object.
Personally I would do this sort of thing with subpolygon displacement instead of using the primitives. In this example I've created a quick circular landscape by way of the layer shader, which works similar to PS.
http://www.3danvil.com/tutorials/circular%20mtns%20displace.c4d