If you have ever tried to make big things that require they be made in parts, you have probably run into this problem: the join points show. Look at the example in the lower right of the image (2 arrows). The desired result is to the left (single arrow).
This is not a matter of getting the vertices to match. Even if they do match perfectly, you get shading errors in Second Life. This is a problem with vertex normals.
There is an ongoing thread about this problem in the SL Forum: Visible seam when connecting two smooth shaded pieces. A good explanation of how to achieve a smooth surface at the connection point with correct shading is given by Drongle McMahon here.