I am extending the standard event.booth module with my custom module. I am trying to add pages to the "Registrations" Notebook the event.booth form has. Unfortunately tho, i cant select the existing notebook with xpath because it also gets added through a second module (called event_booth_sale).

Here are my attempts

<record id="view_booth_form_inherit" model="ir.ui.view">
    <field name="name">event.booth.inheritfield>
    <field name="model">event.boothfield>
    <field name="inherit_id" ref="event_booth.event_booth_view_form_from_event">field>
    <field name="priority">20field>
    <field name="arch" type="xml">
        <xpath expr="//sheet" position="inside">
            <notebook>
                <page string="Allgemein">
                    <group>
                        ...
                    group>

                page>
                <page string="Wände">
                    <group col="4">
                        ...
                    group>
                page>
            notebook>
        xpath>
        <xpath expr="//sheet" position="inside">
            <field name="plan" widget="image"/>
        xpath>
    field>
record>

This also doesnt work:

<record id="view_booth_form_inherit" model="ir.ui.view">
    <field name="name">event.booth.inheritfield>
    <field name="model">event.boothfield>
    <field name="inherit_id" ref="event_booth.event_booth_view_form_from_event">field>
    <field name="priority">20field>
    <field name="arch" type="xml">
        <xpath expr="//notebook" position="inside">
              <page string="Allgemein">
                    <group>
                        ...
                    group>

                page>
                <page string="Wände">
                    <group col="4">
                        ...
                    group>
                page>
            notebook>
        xpath>
    field>
record>

This is almost always the result:

As you can see, there are two notebooks instead of one. I dont know what to do in order to combine them.

I also tried adding 

groups="base.group_no_one"

to the notebook, same results.

Does anyone know how to do this?