A script I wrote after my old one broke to mirror blendshapes. This one seems to work a lot faster as well, which is always a good thing.

load the things into the right place, and run it.

duplicate your base mesh
place the base mesh at 0,0,0 or  any other place, just they all have to be at the same place

run it


it works by
duplicating the base, scale it -1 in X
apply the blendshape
apply a wrap deformer from the base mesh to the new mesh
turn on the blendshape
set the new mesh scale X to 1 again


since wrap deformers take quite a bit of memory it may or may not work with denser meshes.