I prefer to do it manually but I'm weird like that. I do a ton of post transform or difference stuff and then bake down to a FBM. Blender's very precise proportional editing falloff is tremendously helpful with numerically exact transformations from one side and then applied to the other Obviously motion or scaling morphs need to be applied individually to their own particular origin point. A single origin derived scale won't work properly with eyes for instance, upon splitting the result it's not scaling from it's own origin but from an average if you see my meaning. So too with boob motions etc. Actually boobs are another set of procedures lol.

Anyway, after that probably pointless and confusing blathering...

ADP has a script for flipping left to right etc. Works pretty good. Here's the thread with the script...

https://www.renderosity.com/forums/threads/2963260/is-there-a-way-to-mirror-a-morph