One way is to just not mirror stuff. Or just mirror stuff that doesn't cross the center plane.
Another is to use a curved reference surface instead of a flat reference surface. Don't make a surface c2 to a flat surface because you'll get a flat strip, that should be self-evident.
Another thing to watch out for is mirroring splines within a sketch. These can be really tricky to make sure they're really symmetrical. I would recommend mirroring the spline across the centerline to make sure the mirror lies exactly on top of the original. You can get in a lot of trouble with this, even when it looks good by eye. Splines can get some internal residual tension, such that all the points are symmetrical, but the resulting spline is not.
Hello Ryan,.. yes, in general, C2 at the center is not always good... and it's a good subject which needs to be brought up over and over again... I have to constantly check this during and after and of course as a final check before releasing the data to be cut! (I got burnt one time... major ouch!)
btw,.. I usually combine both the curvature and zebra to get as much information on deviation/distortion as possible.
Build half of the surface to the symmetry plane. Maintain the edges normal to the plane. If you do this you will have C2 continuity.