Cynthia,
If the project in question is on Project Server 2002, you (or the Project
Admin) would need to create a custom category just for this project (under
Admin > Manage Security), add the two users to the category (under Available
Users and Groups), and then add the project (under Available Projects)--plus,
make sure to carefully review the rest of the many settings on this page.
Keep in mind that if you have a lot of situations where this is necessary
(as I do), it can turn into a lot of admin work.
--Marcus Pierce