Complex sentence: When zookeepers realized that Roy and Silo were both male, it occurred to them to give them the second egg of a mixed-sex penguin couple, a couple which had previously been unable to successfully hatch two eggs at once.
Split into simpler sentences: When zookeepers realized that Roy and Silo were both male, it occurred to them to give them an egg to hatch. A second egg was obtained from a male-female penguin couple which had previously been unable to successfully hatch two eggs at once.