Addition to Live2D_Preprocess.jsx. Split processed layers with vertical symmetry. Your group/layer(s) must have the "__sym" suffix in order for them to split.

Live2D_Preprocess.jsx

// **************************************************************************************
// This is Javascript for Adobe Photoshop CC
// Copyright (c) 2018 Live2D inc.
// This software is released under the MIT License.
// https://opensource.org/licenses/mit-license.php
//
// 「*」(アスタリスク)が付いていないレイヤーセットを結合
// 更新 2019/04/04  クイックマスクモードになっていると結合できない問題を解消
// **************************************************************************************
(function (){


    var Layers = activeDocument.layers;

    activeDocument.suspendHistory("フォルダを結合","LayerCheck(Layers)");
    activeDocument.suspendHistory("Split Symmetry","SplitSymmetry(Layers)");

function SplitSymmetry (Targets) {

    var docRef = app.activeDocument;
    for (var i = 0 ; i < Targets.length ; i++){
        var SelectLayer = Targets[i];
        var nameSymIdx = (SelectLayer.name).toLowerCase().lastIndexOf("__sym");
        if (nameSymIdx !== -1){
            SelectLayer.allLocked = false;
            var layerNameBase = SelectLayer.name.substring(0, nameSymIdx);
            docRef.activeLayer = SelectLayer;

            var bounds = SelectLayer.bounds;
            var leftR = bounds[0];
            var topR = bounds[1];
            var widthR = (bounds[2] - bounds[0]) / 2;
            var heightR = bounds[3] - bounds[1];
            if (widthR === "0 px" || heightR === "0px") {
                alert ("skipping " + layerNameBase + ": Width or Height is 0");
                continue;
            }
            var selectRegionL = Array(  Array(leftR, topR),
                                        Array(leftR + widthR, topR),
                                        Array(leftR + widthR, topR + heightR),
                                        Array(leftR, topR + heightR),
                                        Array(leftR, topR));


            var leftL = widthR;
            var topL = topR;
            var widthL = bounds[2] - bounds[0];
            var heightL = heightR;
            if (widthR === "0 px" || heightR === "0px") {
                alert ("skipping " + layerNameBase + ": Width or Height is 0");
                continue;
            }
            var selectRegionR = Array(  Array(leftL, topL),
                                        Array(leftL + widthL, topL),
                                        Array(leftL + widthL, topL + heightL),
                                        Array(leftL, topL + heightL),
                                        Array(leftL, topL));

            
           
            docRef.activeLayer = SelectLayer;
            try {     
                docRef.selection.select(selectRegionR);
                docRef.selection.copy();
                docRef.selection.clear(selectRegionL);
                var newLayerR = docRef.artLayers.add();
                docRef.activeLayer = newLayerR;
                newLayerR.name = layerNameBase + "_R";
                pasteInPlace();
                docRef.selection.deselect();
                newLayerR.move(SelectLayer, ElementPlacement.PLACEAFTER);
                SelectLayer.name = layerNameBase + "_L";
            }
            catch (e) {
                alert ("error running split on " + layerNameBase + ": " + e.message);
            }

        }
    };
}

function pasteInPlace(enabled, withDialog) {
    cTID = function(s) { return app.charIDToTypeID(s); };
    sTID = function(s) { return app.stringIDToTypeID(s); };
    if (enabled != undefined && !enabled) return;
    var dialogMode = (withDialog ? DialogModes.ALL : DialogModes.NO);
    var desc1 = new ActionDescriptor();
    desc1.putBoolean(sTID("inPlace"), true);
    desc1.putEnumerated(cTID('AntA'), cTID('Annt'), cTID('Anno'));
    executeAction(cTID('past'), desc1, dialogMode);
};

function LayerCheck(Targets){
    //  ===  パスの削除処理 start  =======
    var docRef = app.activeDocument;
    var PathItems = docRef.pathItems;
    var numPath = PathItems.length;
    for(var i = 0; i < numPath;i++){
        var PathItem = PathItems[0];
        PathItem.remove();
        }
    //  ===  パスの削除処理 end  =======
    var qmask = docRef.quickMaskMode;
    if(qmask == true){
        docRef.quickMaskMode = false;
        }
    
    var Vflag = false;
    for (var i = 0 ; i < Targets.length ; i++){
        var SelectLayer = Targets[i];
        var LayerType = SelectLayer.typename;
        if(SelectLayer.visible == false){
            Vflag = true;
            }
        activeDocument.activeLayer = SelectLayer;
        if(LayerType == "LayerSet"){
            if(SelectLayer.name.match(/\*/g) != null ){
                if(Vflag == true){
                    activeDocument.activeLayer.visible = false;
                    }
                 LayerCheck(SelectLayer.layers);//  「*」付きのレイヤーセットの場合は再帰処理
                }
            else{
                activeDocument.activeLayer.merge();
                }
            }
        
        if(activeDocument.activeLayer.typename == "ArtLayer" && hasLayerMask() == true){
            if(MaskEnabled() == true){
                applyLayerMask();
                }
            else{
                selectLayerMask();
                deleteMask();
                }
            }
        if(Vflag == true){
            activeDocument.activeLayer.visible = false;
            Vflag = false;
            }
        }
    };

// === レイヤーマスクがあるかどうかの判定
function hasLayerMask() {
    var hasLayerMask = false;
    try {
        var ref = new ActionReference();
        var keyUserMaskEnabled = app.charIDToTypeID( 'UsrM' );
        ref.putProperty( app.charIDToTypeID( 'Prpr' ), keyUserMaskEnabled );
        ref.putEnumerated( app.charIDToTypeID( 'Lyr ' ), app.charIDToTypeID( 'Ordn' ), app.charIDToTypeID( 'Trgt' ) );
        var desc = executeActionGet( ref );
        if ( desc.hasKey( keyUserMaskEnabled ) ) {
            hasLayerMask = true;
            }
        }catch(e) {
            hasLayerMask = false;
            }
        return hasLayerMask;
        };

//  レイヤーマスクの適用
function applyLayerMask(){
    try{
    var idDlt = charIDToTypeID( "Dlt " );
    var desc78 = new ActionDescriptor();
    var idnull = charIDToTypeID( "null" );
    var ref29 = new ActionReference();
    var idChnl = charIDToTypeID( "Chnl" );
    var idChnl = charIDToTypeID( "Chnl" );
    var idMsk = charIDToTypeID( "Msk " );
    ref29.putEnumerated( idChnl, idChnl, idMsk );
    desc78.putReference( idnull, ref29 );
    var idAply = charIDToTypeID( "Aply" );
    desc78.putBoolean( idAply, true );
    executeAction( idDlt, desc78, DialogModes.NO );
    }catch(e){};
    };

//  レイヤーマスクが有効か無効か判定
function MaskEnabled(){
    var ref = new ActionReference();  
    ref.putProperty(charIDToTypeID('Prpr'),stringIDToTypeID("userMaskEnabled"));  
    ref.putEnumerated(charIDToTypeID("Lyr "), charIDToTypeID("Ordn"), charIDToTypeID("Trgt"));
    var maskFlag = executeActionGet(ref).getBoolean (stringIDToTypeID("userMaskEnabled"));
    return maskFlag;
}

//  レイヤーマスクを削除
function deleteMask(){
    var idDlt = charIDToTypeID( "Dlt " );
    var desc60 = new ActionDescriptor();
    var idnull = charIDToTypeID( "null" );
    var ref28 = new ActionReference();
    var idChnl = charIDToTypeID( "Chnl" );
    var idOrdn = charIDToTypeID( "Ordn" );
    var idTrgt = charIDToTypeID( "Trgt" );
    ref28.putEnumerated( idChnl, idOrdn, idTrgt );
    desc60.putReference( idnull, ref28 );
    executeAction( idDlt, desc60, DialogModes.NO );
}

//  =====  レイヤーマスクがあった場合に選択する関数
function selectLayerMask() {
    try{ 
        var id759 = charIDToTypeID( "slct" );
        var desc153 = new ActionDescriptor();
        var id760 = charIDToTypeID( "null" );
        var ref92 = new ActionReference();
        var id761 = charIDToTypeID( "Chnl" );
        var id762 = charIDToTypeID( "Chnl" );
        var id763 = charIDToTypeID( "Msk " );
        ref92.putEnumerated( id761, id762, id763 );
        desc153.putReference( id760, ref92 );
        var id764 = charIDToTypeID( "MkVs" );
        desc153.putBoolean( id764, false );
        executeAction( id759, desc153, DialogModes.NO );
        }
        catch(e){

        }
    }


})();