My youngest started pre school yesterday. She came home all excited with some wonderful pictures, then she got really serious and kind of upset. She looked right into my eyes and said, "but we didn't learn how to read today."
My son chirped up that it was preschool and they don't teach you how to read. She looked hurt, but he quickly made up for it by saying "But I'll teach you how to read."
The youngest will feel they are being left behind as the older one begins to learn new things. By encouraging the older one to use his new found skills to either read stories to the younger child or teach the younger child what they are learning, both children will get something out of their time together.