Multiple Field Groups on Post?

Not sure about the user restrictions, but you can indeed have multiple field groups at a time on a post. Several of my personal setups use 2 field groups, one in the main area and one in the sidebar. But placement does not matter.

If you are using “User” from the dropdown, change this to “Logged in User Type” and select Administrator as the user type and you should see it only if logged in as admin. Do the same for the editor level. Hope this helps!