The seam in the middle normally get´s solved at th eend of modelling.
When you combine the two halves, check for normals direction on both -flip the offending one if needed-, and finally Merge Vertices on the whole object with a 0,0001 tolerance.

If the middle edge isn´t that pronounced, then this should take care of it, if it still doesn´t suit you then you´ll have to tweak manually those middle vertices.

Hope this helps,