Great thread! How often has (a) a winner of both not won the USO or (b) a winner of neither won the USO?
Only Agassi '95, Rafter '98, and Roddick '03 have won the Canada-Cincy double.
So, only Agassi '95 has won both and not won the USO.
For the second one...probably rather often. In fact, three straight years: Fed '08, Delpo '09 and Nadal '10, to begin with.
Also Sampras '02, Hewitt '01, Agassi '99, Rafter '97...we could go on. If my (poor) math is right, the 43 times the USO has been held, 26 times has the men's winner won neither Cincy nor Canada that year.
EDIT: Also Sampras '96, Sampras '95, Sampras '93, Edberg '92, Edberg '91, Sampras '90, Becker '89, Lendl '86, Lendl '85, Connors '83, Connors '82, McEnroe '80, McEnroe '79, Vilas '77, Connors '76, Connors '74, Newcombe '73, Laver '69, Ashe '68.
So as you can see, it's actually pretty damn common to win the USO while winning neither Canada nor Cincy.