Currently only one tab can be set to default regardless of userrole. This is simply the limitation of the tabcontainer widget as it is. So I'd work around it with either multiple forms or tweaking the visibility as you mentioned already, or if reshuffling the tabs is no problem I'd stick to that.
Benjamin, this is a recent issue in the technology. Currently in Mx5, regardles of what you configure, the platform will never open the tab pages as the default page, if they have conditional visibility specified.
The behavior used to be that the browser tries to open the 'default' tab page. If no default tab page is specified it will open the first tab.
If the tab that is to be opened is hidden, it will open the next tab.
Right now a tab page that has conditional visibility specified can never be opened by default.