This code bridges two incoming calls together. With this minimal conferencing attribute setup, no sound is played, participants can speak as soon as they join, and the conference ends immediately when a participant hangs up. 
from plivo import plivoxml

response = plivoxml.ResponseElement()
response.add(
    plivoxml.ConferenceElement(
        'My Room',
        enter_sound="",
        start_conference_on_enter=True,
        end_conference_on_exit=True))
print(response.to_string())
Response
<Response>
	<Conference enterSound="" startConferenceOnEnter="true" endConferenceOnExit="true">My Room</Conference>
</Response>