I'm not sure if it HAS happened before.
The Buckeyes went from Woody (1951 - 1978) to Earle Bruce (1979-1987) to Cooper (1988 - 2000 ) to Tressel ( 2001 - 2010) and Fickell, thats it.
60 years, 4 coaches, thats pretty good I think !
As for Michigan,
Bo went from 1969 to 1989, Gary Moeller from 1990 to 1994, then LLLLLLLoyd Carr from 1995 to 2007, then Rich Rod 2008 to 2010 and now Brady Hoke.
So, in the current times (since Woody and Bo anyways) the teams have never been without coaches in the same years.