Hi Brian, one technique to get a median shape between 2 curves is to do a Construct > Loft between the 2 curves and then extract an isoparm (Construct > Curve > Iso) from the loft surface.

I think Max's cmorph plugin does that. http://moi3d.com/wiki/MaxScriptArchive

But that's only going to work for pretty simple cases, not if there is something like a centerline for a complex boundary with multiple branches. Another keyword you might search for is "straight skeleton".

- Michael