I recall that in the early 1990's, several broadcast and cable networks simulcast an animated cartoon special featuring nearly all of the top cartoon characters of the era (and a few "legends") in a show whose purpose, IIRC, was to fight drug abuse.
More recently, in December of 2007, CBS and NBC simulcast the NFL Network's coverage of the final regular-season game of the New England Patriots (they played the New York Giants at the old Meadowlands stadium) on a Saturday night since New England was going for a perfect 16-0 regular-season (they got the 16-0 record, but lost to those same Giants a few weeks later in the Super Bowl).
As a result of that situation, the NFL altered it's scheduling procedures so that all 16 games on the final weekend of the regular-season are scheduled for Sunday afternoon, with one of them moved to prime-time the prior Monday. Had that procedure been in place in 2007, the New England/New York Giants game would have been "flexed" to Sunday night from Sunday afternoon for a full national telecast on NBC.