SqlServer: can one table's PK be the FK of other two table's? I have three tables project, users and proj_user. 
The PK of proj_user table is the FK of project and users table's pk e.g.(project_id,user_id)

My question is, is it possible for the project_id and user_id's data to be different, like?:
project_id = '12345' and user_id = 'abc987'



Edit Thanks for the quick response. But just look at this scenario

TABLE project (
    project_id PRIMARY KEY
    ...
)

TABLE users (
    user_id PRIMARY KEY
    ...
)

TABLE proj_user (
proj_user_id
project_id
user_id
PRIMARY KEY (proj_user_id),
FOREIGN KEY (proj_user_id) REFERENCES projects (project_id),
FOREIGN KEY (proj_user_id) REFERENCES projects (user_id)
)


I want the project_id and user_id values to be different.

What do I need to change here?
If I read your question correctly, you're describing the following:

TABLE project (
    project_id PRIMARY KEY
    ...
)

TABLE users (
    user_id PRIMARY KEY
    ...
)

TABLE proj_user (
    project_id,
    user_id,
    PRIMARY KEY (project_id, user_id),
    FOREIGN KEY (project_id) REFERENCES projects (project_id),
    FOREIGN KEY (user_id) REFERENCES users (user_id)
)


... which is entirely valid, yes.