[Observium] observium php session timeout affecting another application?

i'm running two php applications on a LAMP machine. one is cacti, the other is observium. i'm using name-based virtual hosting with one IP address. when i have just the cacti virtual host running, i can stay logged in to cacti forever, i.e. my session never times out. when i enable the observium virtual host, cacti prompts me for my login credentials again after 1-2 minutes. i'm using LDAP authentication for cacti and the built-in authentication for observium (user added via adduser.php). any idea why that would be?