Strange but true fact: Kentucky Derby winners rarely face each other. I've been struggling to think of the last time we saw two Kentucky Derby winners in the same race, and can't think of a time it's happened in the last 18 years. The main reason is that most Derby winners don't run as 4 year olds--only 4 of the last 12 ran as an older horse, and one of them (Monarchos) ran only once.
The most notable meeting of Derby winners since the start of the Breeders Cup was the 1987 Classic, which squared off '86 winner Ferdinand and '87 winner Alysheba. This wasn't the last time they met, but it was the most memorable.