Useful for breaking a symmetrical blend target into a smoothly blending left/right, up/down (whatever/whatever) pair.
The base object will be your character's head (or whatever) in its default expression, and the target object will be a modified duplicate of the same mesh that you want to split into two asymmetrical meshes. There can only be one set of blend joints per scene, but you can weight multiple Base Objects to them.
Duplicate your (unskinned) base object and then run abTwoFace at the command line. The taper value determines the width of the transition between the joint weights as a percentage of the width (x dimension) of the base object (or the guide object, if one is selected). Increase the value to widen (and smooth) the transition between the left and right joint weights. Leave the "Follow Guide Object" textfield empty for now.
Now, select the duplicated object and click "Create Weighted Base Object". Once created, you should have two joints, abLeftBlendJNT and abRightBlendJNT, on your newly weighted base object. If you want to adjust the weight taper on an existing base object, just enter a new value in the field and click the "Taper Weights" button.
The "Follow Guide Object" is useful if you have, say, a separate mouth mesh for which you want to create blends that follow your head mesh. The weighting will follow that of the Guide Object (assuming the same taper value was used for each mesh). The result mesh will be placed relative to its Guide result meshes. Clear this field for default (no guide object) behavior.
Once you're happy with the weighting, select your newly weighted mesh and click "Select Base Object". Now select your target mesh (the blend target that you want to split into two meshes). Click "Swap Left/Right Names" if you, like me, have an object facing -z -- it swaps the naming on the result objects. The left object will now be prefixed with an "rt_" and vice versa.
Now just click "Create Meshes" to create the new objects, and you're off...
note: To change your default naming prefixes, search the script for one of the following two strings:
string $lfDefPrefix = "lf_"; string $rtDefPrefix = "rt_";
and change their values ("lf_" and "rt_") to your desired left and right prefixes.