Complex sentence: Tapper was passed over as candidate to replace George Stephanopoulos as anchor of "This Week" when Stephanopoulos was chosen to replace Diane Sawyer as co-host of "Good Morning America" when she assumed the role as anchor of "World News".
Split into simpler sentences: Tapper was passed over as candidate to replace George Stephanopoulos as anchor of "This Week". Stephanopoulos took over for Diane Sawyer, co-host of "Good Morning America", when she assumed the role as anchor of "World News".