Hi Brian, the main way that comes to mind is using Flow - that does allow you to map one envelope curve offset from an existing curve path.

I'm not really sure what you are asking about exactly though, do you want to form a kind of median curve between 2 existing curves?

One possibility for that is to do a loft between the 2 curves and then extract an isoparm (Construct > Curve > Iso) from that lofted surface.

- Michael