SqlServer: can one table's PK be the FK of other two table's?
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.