partner-contact/partner_second_lastname/views/res_partner.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2015 Tecnativa - Jairo Llopis
     Copyright 2017 Tecnativa - Pedro M. Baeza
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
<odoo>
    <record id="partner_simple_form" model="ir.ui.view">
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="partner_firstname.view_partner_simple_form_firstname"/>
        <field name="arch" type="xml">
            <data>
                <xpath expr="//field[@name='firstname']" position="attributes">
                    <attribute name="attrs">{'required': [('lastname', '=', False), ('lastname2', '=', False), ('is_company', '=', False)]}</attribute>
                </xpath>
                <xpath expr="//field[@name='lastname']" position="attributes">
                    <attribute name="attrs">{'required': [('firstname', '=', False), ('lastname2', '=', False), ('is_company', '=', False)]}</attribute>
                </xpath>
                <xpath expr="//field[@name='lastname']" position="after">
                    <field name="lastname2" attrs="{'required': [('firstname', '=', False), ('lastname', '=', False), ('is_company', '=', False)]}"/>
                </xpath>
            </data>
        </field>
    </record>

    <record id="partner_form" model="ir.ui.view">
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="partner_firstname.view_partner_form_firstname"/>
        <field name="arch" type="xml">
            <data>
                <!-- Main form -->
                <xpath expr="//field[@name='firstname']" position="attributes">
                    <attribute name="attrs">{'required': [('lastname', '=', False), ('lastname2', '=', False), ('is_company', '=', False)]}</attribute>
                </xpath>

                <xpath expr="//field[@name='lastname']" position="attributes">
                    <attribute name="attrs">{'required': [('firstname', '=', False), ('lastname2', '=', False), ('is_company', '=', False)]}</attribute>
                </xpath>
                <xpath expr="//field[@name='lastname']" position="after">
                    <field name="lastname2" attrs="{'required': [('firstname', '=', False), ('lastname', '=', False), ('is_company', '=', False)]}"/>
                </xpath>
                <!-- Inner contact form of child_ids -->
                <xpath expr="//field[@name='child_ids']/form//field[@name='firstname']" position="attributes">
                    <attribute name="attrs">{'required': [('lastname', '=', False), ('lastname2', '=', False), ('is_company', '=', False)]}</attribute>
                </xpath>
                <xpath expr="//field[@name='child_ids']/form//field[@name='lastname']" position="attributes">
                    <attribute name="attrs">{'required': [('firstname', '=', False), ('lastname2', '=', False), ('is_company', '=', False)]}</attribute>
                </xpath>
                <xpath expr="//field[@name='child_ids']/form//field[@name='lastname']" position="after">
                    <field name="lastname2" attrs="{'required': [('firstname', '=', False), ('lastname', '=', False), ('is_company', '=', False)]}"/>
                </xpath>
            </data>
      </field>
    </record>

</odoo>