partner-contact/partner_multi_name/views/res_users_views.xml

<?xml version="1.0" encoding="utf-8"?>
<flectra>
    <record id="users_form" model="ir.ui.view">
        <field name="model">res.users</field>
        <field name="inherit_id" ref="partner_second_lastname.users_form"/>
        <field name="priority">100</field>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='firstname']" position="replace"/>

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

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

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