This is a Blender add-on (my first ever).
this add-on helps you with saving time. It can split, join or mirror shape keys.
.L to .R (and vice versa)
.l to .r (and vice versa)
_L to _R (and vice versa)
_l to _r (and vice versa)
The Falloff function can be slow if used on a dense mesh, because it has to go trough every vertex in the selected range and calculate its "blend from shape value". If the value of the range or strength is set to 0 it skips the whole calculation part therefore it's faster.
By default the plugin panel is spawned at the bottom. You can just drag it up nearer to the Shape Keys panel if you want to by holding on to the dotted symbol on the right side of the panel and dragging.
It was made for 2.8b but it should work for 2.79 also.