I'm starting with a Bezier curve:
I convert it to a mesh, then duplicate it and mirror the duplicate on the X axis, then join the two meshes. Then I duplicate and mirror again, now on the Y axis, and join the two halves, and I get this:
From there, in the Edit Mode, I hit 'F' to create a face and I get half a face: If I hit 'F' a second time, it makes a second face: My guess is that I'm doing something wrong when I join the different sections together so somehow it doesn't look like one long mesh. I've also done this where I've created the entire curve, all 4 quarters joined together, then extruded it along the Z-axis so it became a prism. I joined the faces on one end of the prism and still got two faces.
What am I doing wrong that all the vertices in the joined shape do not act like one set of vertices, but, instead, act as two sets? And, if there's no way to make it produce one face, what do I need to do to join the two faces into one? (I'm also open to doing more of this operation before I convert the curve to a mesh, but last I checked, there was no way to join two curves.)