You can use the same DB, but different schema or different user. You can also use the same schema as long as table names (and other objects names) din't match.
I recomend to use diferent schemas for different web apps.
Camilo.
It seems that using 2 different databases will cause duplicate schema (tables) and duplicate data. So in your case I prefer to change my mind and recomend you to use a single database. In general I don't see any problem with that.
Regards,
Camilo.