Penn State and Nebraska weren't in the Big Ten at the time, but here's a look at a few weeks in 1985:
9/24: 3. Iowa, 5. Ohio State
10/1: 1. Iowa, 5. Ohio State
10/8: 1. Iowa, 3. Michigan
10/15: 1. Iowa, 2. Michigan
10/22: 1. Iowa, 3. Penn State, 4. Michigan, 5. Nebraska
10/29: 1. Iowa, 3. Penn State, 4. Michigan, 5. Nebraska
11/5: 2. Penn State, 3. Nebraska, 4. Ohio State
11/12: 1. Penn State, 2. Nebraska, 3. Ohio State, 5. Iowa
11/19: 1. Penn State, 2. Nebraska, 3. Iowa
11/26: 1. Penn State, 2. Iowa, 5. Michigan
12/3: 1. Penn State, 3. Iowa, 5. Michigan
Final: 2. Michigan, 3. Penn State
I'll do more research and see if I can find a time when three schools that were simultaneously in the Big Ten have ever been ranked in the AP top-5 in the same week.