I used 2 parts to create a blend, then removed one of the parts. Here's an example with the 2 surfaces. After blending, I deleted the bottom surface, then miirored the 2 top ones down. They can be connected and joined. For your example I used a "loft" because your model looks straight on the sides.. You could connect them with blend also, for a smoother look.

I think in the example I posted I trimmed the blend up from the bottom a little too, before I mirrored it.