partner-contact/partner_second_lastname/views/res_user.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
     © 2015 Grupo ESOC Ingeniería de Servicios, S.L.U. -->

<openerp>
<data>

<!-- Required before modifying `base.vew_users_form`.
     https://github.com/odoo/odoo/issues/6324#issuecomment-93534579 -->
<function model="res.groups" name="update_user_groups_view" />

<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>

</data>
</openerp>