Version 3.0 of the Ventrilo server does not allow multiple logins from a single IP using a single user account by default. Since most of the time people log in using the Guest account, this can be annoying. To change this behavior, log in as an admin through your Ventrilo client, open up the user editor, select the account that both users are using (such as the Guest account), select the network tab, and change Dup IPs to No Limit. You can also fix this by creating a separate account for each user (which is the preferred solution).
The version 3.0 FAQ in the Ventrilo support forums covers this and also some other common questions: http://www.ventrilo.com/forums/showthread.php?t=23397