The best way is to use ERC to slave the eyes to the channel that contains the morph. For details of doing this, see the thread "A Compendium of PZ2 Techniques".

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2735605

You don't need to read the thread from the start, it is quite long. Read the posts titled "The Monster Eyes Problem (part 1)." and "The Monster Eyes Problem (part 2).", they are quite a long way into the thread.  After that, you may also want to look at some of the proceeding posts from "Injecting ERC slaving (part 1)." onwards, which also relate to the subject.