Multisite, the main and two blogs.
Plugins in the main: Many ... :-)
Plugins in one of the blogs: Members, Multisite User Management and WP Hide Dashboard.
In the main, hides the dashboard perfectly.
In another blog, without any effect.
Recalling that the "Multisite User Management", allows the user to access the dashboard specific blog, not just the main blog.
Do not know if I explained right.