I have solved it some days back. I have created pos-actionbar-button-left and pos-actionbar-button-right instead of pos-actionbar-button-list and appended the button based on some condition. The JavaScript code is as below

    instance.point_of_sale.ActionBarWidget.include({
    add_new_button: function(button_options){
        var button = new instance.point_of_sale.ActionButtonWidget(this,button_options);
        this.button_list.push(button);
        if(button_options.name){
            this.buttons[button_options.name] = button;
        }
        if (button_options.name == 'validation' || button_options.label == "Next Order"){
            button.appendTo(this.$('.pos-actionbar-button-right'));
        }
        else{
            button.appendTo(this.$('.pos-actionbar-button-left'));
        }
        return button;
    },
});