Both $S1$ and $S2$ are FALSE.
In a relation scheme multiple foreign attributes can be present referring to primary keys of other relation schemes. A typical example is an $\text{EXAM_RESULTS(sid,eid,marks)}$ scheme where $\text{sid}$ and $\text{eid}$ are foreign keys referring to the primary keys in $\text{STUDENT}$ and $\text{EXAM}$ schemes respectively.
$S2$ is FALSE because a foreign key can refer to the same scheme (self-referencing foreign key). A typical example is an $\text{EMPLOYEE(eid, mid, $\ldots$)}$ scheme where $\text{mid}$ is the Manager ID referring to the primary key $\text{eid}$ of the same scheme.