You need something like that in your xml that would create two Note tab.

                <page string="First Note">
                    <separator string="'Comment1'" colspan="4"/>
                    <field name="your_field" colspan="4" nolabel="1"/>
                </page>
                <page string="Second Note">
                    <separator string="'Comment2'" colspan="4"/>
                    <field name="your_field_2" colspan="4" nolabel="1"/>
                </page>

New update Something like this but you will have to create a one2many field for the notes this XML below should work.

                <page string="Notes">
                    <field colspan="4" mode="form,tree" name="notess" select="1" height="260"> 
                        <form string="User Note">

                            <field name="notes"/>
                            <field name="note_description" />
                        </form>
                        <tree string="User Note">
                            <field name="notes_id" />
                            <field name="note_description" />                         
                        </tree>
                    </field>
                </page>

and in your python add something like this

class lab_product(osv.osv):
_name = 'lab.product'
_columns = {
    'name': fields.char('Field Name',size=x'),
    'notess': fields.one2many('lab.product.notess','notes_id','User Comment'),
    'note_description': fields.related('notess','note_description',type='char', string='Short Description'),
    'notid': fields.related('notess','notes',type='text', string='Work Note'),
}
 lab_product()

class lab_product_notess(osv.osv):
_name = 'lab.product.notess'
_columns = {
    'notes_id': fields.many2one('lab.product','Product', select=True, required=True),
    'note_description': fields.char('Short Description', size=100),
    'notes': fields.text('Work Note', size=10000),
}
lab_product_notess()

Hope this is helpful. You can always improve on it but that will do what you wanted You can see I created two classes and made one one to many and the other many to one. The lab product class can have many notes and many note class can have one lab product. Relate the field and that should do it. When you run this, you will be able to make lots of notes under Notes tab (as many as you want) you can then set to readonly as you wish.

            <page string="First Note">
                <separator string="'Comment1'" colspan="4"/>
                <field name="your_field" colspan="4" nolabel="1"/>
            </page>
            <page string="Second Note">
                <separator string="'Comment2'" colspan="4"/>
                <field name="your_field_2" colspan="4" nolabel="1"/>
            </page>