Hi Scott - typically if you create a freeform surface it won't come to an exact horizontal tangent direction at the middle line, and that means that when mirrored it will produce the kind of seam like you're seeing there.

Usually to avoid that it's better to make the full shape all in one go of the Network command with closed curves for the profiles rather than half curves. And the closed curve should be smooth as well.

Another possibility is to make a bit of space (either by moving them apart or by trimming out a section using a rectangle drawn in the top as the cutting object) between the halves and then use Construct > Blend to make a smooth connecting piece between the separated pieces.

- Michael