Usually, you can use the fact that you can toggle visiblity on/off, or simply move the object wherever you want, instantly, to solve this (use a combined object, then swap it out for the seperated model). This also allows the seperated pieces to include things like bevels, or smoothed edges. However, this may not be appropriate in places where there is a large amount of rigging involved, and it may make skinning difficult.
If using seperate objects, the best way to deal with the surface normal issue is to make sure that the surface normals are at the same angle at the seam. This is simple to produce (and visualize) with polygons. For example - instead of splitting a sphere on the initial geometry of the primitive, insert an edge loop, and split on that. This will ensure that face normals match on both pieces.