project_task_views.xml

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

        <record id="view_task_form2" model="ir.ui.view">
            <field name="name">view.task.form2.form.inherit</field>
            <field name="model">project.task</field>
            <field name="inherit_id" ref="project.view_task_form2"/>
            <field name="arch" type="xml">
                <xpath expr="//notebook" position="before">
                    <group>
                        <field name="participant_ids" widget="one2many">
                            <tree string="Participants" editable="top">
                                <field name="partner_id"/>
                                <field name="lastname"/>
                                <field name="firstname"/>
                                <field name="commune"/>
                                <field name="mobile"/>
                                <field name="fixe"/>
                                <field name="email" widget="email"/>
                                <field name="is_present"/>
                            </tree>
                        </field>
                    </group>
                </xpath>
                <xpath expr="//field[@name='date_deadline']" position="replace">
                    <field name="formateur_id"/>
                    <field name="duree_jr"/>
                </xpath>
            </field>
        </record>
    </data>
</odoo>