partner-contact/partner_firstname/views/res_user.xml

<flectra>
    <record id="view_users_form" model="ir.ui.view">
        <field name="name">Add firstname and surnames</field>
        <field name="model">res.users</field>
        <field name="inherit_id" ref="base.view_users_form" />
        <field name="arch" type="xml">
            <xpath expr="//field[@name='name']" position="attributes">
                <attribute name="readonly">True</attribute>
                <attribute name="required">False</attribute>
            </xpath>
            <xpath expr="//field[@name='email']" position="after">
                <group>
                    <field name="lastname" required="not firstname" />
                    <field name="firstname" required="not lastname" />
                </group>
            </xpath>
        </field>
    </record>
</flectra>