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). -->
<flectra>

    <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">
            <data>
                <xpath expr="//field[@name='firstname']" position="attributes">
                    <attribute name="attrs">{'required': [('lastname', '=', False), ('lastname2', '=', False)]}</attribute>
                </xpath>

                <xpath expr="//field[@name='lastname']" position="attributes">
                    <attribute name="attrs">{'required': [('firstname', '=', False), ('lastname2', '=', False)]}</attribute>
                </xpath>

                <xpath expr="//field[@name='lastname']" position="after">
                    <field name="lastname2"
                           attrs="{'required': [('firstname', '=', False), ('lastname', '=', False)]}"
                    />
                </xpath>
            </data>
      </field>
    </record>

</flectra>