My 2.9yr old has started preschool this week two mornings a week, he is ok though still a little unsure, trouble is my 1.9yr old screams the place down when I try and leave (he really wants to stay) and he hate being apart from his brother.
I think you will find it will calm down with time. DD hated it when DS started school as she was used to spending much more time with him, but gradually she realised that no DS meant more time alone with Mummy = good thing!
Can you let your younger child know that you will be doing "something fun" after you have dropped off the older? (preferably not in earshot of the older or he might not want to go to pre-school!)