The only meet and greet passes are part of their paid VIP packages. Demi and David have separate VIP events with separate passes. Their VIP events may overlap timewise so I do not know if you can do both.
Demi Lovato VIP's are sold at Ticketmaster (brown box on the right and I do not know if hers includes a ticket)
http://www.ticketmaster.com/event/0100428EC91091F6?artistid=1224909&majorcatid=10001&minorcatid=1
David Archuleta VIP's (what's left of them) are sold at:
http://www.iloveallaccess.com
You select David's picture then enter archie as the password in the new window. 5-star packages ($199) include one ticket and when they sell out of those they sell Upgrades ($125 and you MUST have your own ticket) and when they sell out that's it.