GUI getting in the way of processing...

main();

function main() {
  if (app.documents.length > 1) {
    mWindow();
  } else {
    alert("Open at least two documents!");
  }
}

function mWindow() {
  var w = new Window("dialog");
  w.alignment = "left";
  w.text = "Import text frames from file";

  var st_targetFileName = w.add("statictext");
  st_targetFileName.text = "TARGET file: " + app.activeDocument.name;
  st_targetFileName.alignment = "left";

  var st_filePathTitle = w.add("statictext");
  st_filePathTitle.text = "Select SOURCE file:";
  st_filePathTitle.alignment = "left";

  var dd_openDocuments = w.add("dropdownlist");
  dd_openDocuments.alignment = "fill";
  buildOpenDocumentsList(dd_openDocuments);

  dd_openDocuments.onChange = function () {
    st_SourceFileNoPages.text =
      "SOURCE file: " +
      app.documents.itemByID(dd_openDocuments.selection.id).pages.length;
    btn_import.enabled = 1;
  };

  var divider = w.add("panel");
  divider.alignment = "fill";

  // PAGES PANEL
  var pnl_Pages = w.add("panel");
  pnl_Pages.text = "Pages";
  pnl_Pages.alignChildren = ["left", "top"];
  pnl_Pages.alignment = "fill";
  pnl_Pages.spacing = "12";

  var st_SourceFileNoPages = pnl_Pages.add("statictext");
  st_SourceFileNoPages.text = "SOURCE file: <Unselected>";

  var st_TargetFileNoPages = pnl_Pages.add("statictext");
  st_TargetFileNoPages.text = "TARGET file: " + app.activeDocument.pages.length;

  var btn_import = w.add("button");
  btn_import.text = "IMPORT!";
  btn_import.enabled = 0;

  btn_import.onClick = function () {
    $.writeln("Import button clicked...");
    var mSource = app.documents.itemByID(dd_openDocuments.selection.id);
    var mTarget = app.activeDocument;

    copyTextFrames(mSource, mTarget);
    w.destroy();
  };

  w.show();
}

function buildOpenDocumentsList(list) {
  mDocuments = app.documents.everyItem().getElements();
  for (var i = 0; i < mDocuments.length; i++) {
    if (mDocuments[i].name != app.activeDocument.name) {
      temp = list.add("item", mDocuments[i].name);
      temp.id = mDocuments[i].id; // Add property so we can easily get a handle on the document later
    }
  }
}

function copyTextFrames(source, target) {
  $.writeln("Copy function...");
  var sourceAllTextFrames = source.textFrames;
  var targetAllPages = target.pages;

  var LPageObjectStyle = target.objectStyles.itemByName("QuranScriptLeft");
  var RPageObjectStyle = target.objectStyles.itemByName("QuranScriptRight");

  for (var i = 0, len = sourceAllTextFrames.length; i < len; i++) {
    mCurrentPage = sourceAllTextFrames[i].parentPage.name;
    targetFrame = sourceAllTextFrames[i].duplicate(
      targetAllPages.itemByName(mCurrentPage)
    );

    var leftPage = Number(mCurrentPage) % 2 == 0 ? 0 : 1;

    if (leftPage == 1) {
      targetFrame.applyObjectStyle(LPageObjectStyle);
    } else {
      targetFrame.applyObjectStyle(RPageObjectStyle);
    }
  }
}