partner-contact/partner_firstname/views/res_partner.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <record id="view_partner_simple_form_firstname" model="ir.ui.view">
        <field name="name">Add firstname and lastname</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_simple_form"/>
        <field name="arch" type="xml">
            <data>
                <xpath expr="//field[@name='name']" position="attributes">
                    <attribute name="attrs">{
                        'readonly': [('is_company', '=', False)],
                        'required': [('is_company', '=', True)]
                        }</attribute>
                </xpath>

                <xpath expr="//h1//field[@name='name']/.." position="before">
                    <group attrs="{'invisible': [('is_company', '=', True)]}">
                        <field name="lastname" attrs=
                            "{'required': [('firstname', '=', False),
                            ('is_company', '=', False),
                            ('type', '=', 'contact')]}"/>
                        <field name="firstname" attrs=
                            "{'required': [('lastname', '=', False),
                            ('is_company', '=', False),
                            ('type', '=', 'contact')]}"/>
                    </group>
                </xpath>
            </data>
        </field>
    </record>

    <record id="view_partner_form_firstname" model="ir.ui.view">
        <field name="name">Add firstname and surnames</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form"/>
        <field name="arch" type="xml">
            <data>
                <xpath expr="//field[@name='name']" position="attributes">
                    <attribute name="attrs">{
                        'readonly': [('is_company', '=', False)],
                        'required': [('is_company', '=', True)]
                        }</attribute>
                </xpath>

                <xpath expr="//h1//field[@name='name']/.." position="after">
                    <div class="oe_edit_only">
                        <group attrs="{'invisible': [('is_company', '=', True)]}">
                            <field name="lastname" attrs=
                                "{'required': [('firstname', '=', False),
                                ('is_company', '=', False),
                                ('type', '=', 'contact')]}"/>
                            <field name="firstname" attrs=
                                "{'required': [('lastname', '=', False),
                                ('is_company', '=', False),
                                ('type', '=', 'contact')]}"/>
                        </group>
                    </div>
                </xpath>

                <!-- Modify inner contact form of child_ids -->
                <xpath expr="//field[@name='child_ids']/form//field[@name='name']"
                    position="attributes">
                    <attribute name="attrs">{
                        'readonly': [('is_company', '=', False)],
                        'required': [('is_company', '=', True)]
                        }</attribute>
                </xpath>

                <xpath expr="//field[@name='child_ids']/form//field[@name='name']"
                    position="after">
                    <div class="oe_edit_only" colspan="2">
                        <field name="is_company" invisible="True"/>
                        <group attrs="{'invisible': [('is_company', '=', True)]}">
                            <field name="lastname" attrs=
                                "{'required': [('firstname', '=', False),
                                ('is_company', '=', False),
                                ('type', '=', 'contact')]}"/>
                            <field name="firstname" attrs=
                                "{'required': [('lastname', '=', False),
                                ('is_company', '=', False),
                                ('type', '=', 'contact')]}"/>
                        </group>
                    </div>
                </xpath>
            </data>
        </field>
    </record>
</odoo>