You can set content level security if you have the permissions to 'set permissions'. A prerequisite is also that the project must be unlocked.
Once you publish, navigate the dashboards and for each set the individual permissions adding the relevant user group.
The caveat with this method is that the Sales group will not even know the second dashboard exists.
If you want them to see there is a dashboard but it be blank, you could use row level permissions.-Using a calculated field ISMEMBEROF("Managers") = TRUE on all the sheets of dashboard two, will only allow members of this Tableau Server group to see the content when they login.
Let me know if this helps.
We definatelly will try the first approach. I wouldn't go with the second one because the users will start asking questions why it is empty.
Thanks a lot.