partner-contact/partner_firstname/views/res_user.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>

<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">
        <data>
            <xpath expr="//field[@name='name']" position="attributes">
                <attribute name="readonly">True</attribute>
            </xpath>

            <xpath expr="//field[@name='email']" position="after">
                  <div>
                    <label for="lastname" string="Lastname"/>
                    <field name="lastname" attrs=
                        "{'required': [('firstname', '=', False)]}" class="oe_inline"/><br/>
                    <label for="firstname" string="Firstname"/>
                    <field name="firstname" attrs=
                        "{'required': [('lastname', '=', False)]}" class="oe_inline"/>
                  </div>
            </xpath>
        </data>
  </field>
</record>

</data>
</openerp>