Thank you, I forgot about this possibility. I have tried to do this, and it is working. If you are interesting, this is example of code (it is data from crm module. Report contains phonecalls, meeting and lead events on one calendar view):

report.py:

from osv import fields,osv
import tools

class crm_summary(osv.osv):
<tab indent must be started here>
_name = "crm.summary"
_auto = False

_columns = {
    'type': fields.char('Type', size=128, readonly=True),
    'name': fields.char('Name', size=512, readonly=True),
    'date': fields.datetime('Date', readonly=True),
}

_order = "date DESC"

def init(self, cr):
    tools.sql.drop_view_if_exists(cr, 'crm_summary')
    cr.execute("""
        CREATE view crm_summary as
          (SELECT l.id || 'l' as id,
          'lead' as type,
          l.title_action as name,
          l.date_action as date
          FROM crm_lead l
          GROUP BY l.id, type, l.title_action, l.date_action
          )
          UNION
          (SELECT c.id || 'c' as id, 
          'call' as type,
          c.name as name,
          c.date as date
          FROM crm_phonecall c
          GROUP BY c.id, type, c.name, c.date
          )
          UNION(
          SELECT m.id || 'm' as id, 
          'meeting' as type,
          m.name as name,
          m.date as date
          FROM crm_meeting m
          GROUP BY m.id, type, m.name, m.date
          )

    """)
<tab indent must be ended here>
crm_summary()

report.xml:

<record id="crm_summary_calendar" model="ir.ui.view">
        <field name="name">Crm summary calendar</field>
        <field name="model">crm.summary</field>
        <field name="type">calendar</field>
        <field name="arch" type="xml">
            <calendar string="Crm summary" color="type" date_start="date">
                <field name="name"/>
            </calendar>
        </field>
    </record>

Have a look at the app https://apps.odoo.com/apps/modules/9.0/joint_calendar/

Building on @duh386 answer.

Ids would overlap, here's my solution:

"""CREATE VIEW my_summary AS
        SELECT *, row_number() OVER (ORDER BY name ASC) AS id
        FROM 
        (
            SELECT
                foo.name AS name,
            FROM
                foo                
        UNION
            SELECT
                bar.name AS name,                 
            FROM
                bar                
        )
        AS
            summary
        """ 

You can use Super_calendar module. you can see it here. https://launchpad.net/server-env-tools