Hello Sebastian,
Thank you for your post.
You can specify the user in connection properties, on the Security tab, see "Send user name" field in the additional group.
Can I connect to the same time to different users?
Yes, you can. Just create two address book records with two different users.
Don't forget to create users and set permissions first. See Host settings -> Security -> Authorization tab (link Users and Access Control).
Hope that helps.
