partner-contact/partner_second_lastname/views/res_user.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2015 Grupo ESOC Ingeniería de Servicios, S.L.U. - Jairo Llopis
     Copyright 2017 Tecnativa - Pedro M. Baeza
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
<odoo>

    <record id="users_form" model="ir.ui.view">
        <field name="name">Add second last name</field>
        <field name="model">res.users</field>
        <field name="inherit_id" ref="partner_firstname.view_users_form"/>
        <field name="arch" type="xml">
            <field name="firstname" position="attributes">
                <attribute name="attrs">{'required': [('lastname', '=', False), ('lastname2', '=', False)]}</attribute>
            </field>
            <field name="lastname" position="attributes">
                <attribute name="attrs">{'required': [('firstname', '=', False), ('lastname2', '=', False)]}</attribute>
            </field>
            <field name="lastname" position="after">
                <field name="lastname2" attrs="{'required': [('firstname', '=', False), ('lastname', '=', False)]}" />
            </field>
      </field>
    </record>

</odoo>