Keep agi running in parallel with MeetMe

I have encountered a problem in keeping agi working, when MeetMe has launched.

What I want to do is:
There will be 2 attendees who will join the conference room.
When the first attendee comes, his credit allows 30 minutes call. Later when the 2nd attendee join the same room, his credit has to be shared with the 1st attendee, i.e. 15 minutes allowed.

However how can I control the 1st attendee hangup time, to update to 15 minutes when 2nd attendee comes? which 1st attendee has already turned on the conference bridge (opened Meetme).

I have tried to use MeetMe -b , to run agi at the background. However, its use has some limitation, which I found the set auto hangup can not be ran.

Please advise. Thanks!