Subdiv Surfaces >
Mirror
Turn on the axes you want to mirror the surface along and click Mirror or Apply. By default, Maya duplicates and flips the surface along the X axis.
If you transform the surface and then freeze the transformation ( Modify > Freeze Transformation) before mirroring, the surfaces will mirror relative to the axis of choice.
If you intend to merge surfaces after mirroring them, line up the original surface to the origin, freeze the transform, then perform the mirror.
