SQL> create table tbl1(
  2                    x number,
  3                    y number
  4                   )
  5  /

Table created.

SQL> create table tbl2(
  2                    y number,
  3                    x number
  4                   )
  5  /

Table created.

SQL> alter table tbl1
  2    add constraint tbl1_pk
  3      primary key(x)
  4  /

Table altered.

SQL> alter table tbl2
  2    add constraint tbl2_pk
  3      primary key(y)
  4  /

Table altered.

SQL> alter table tbl1
  2    add constraint tbl1_fk1
  3      foreign key(y)
  4      references tbl2
  5  /

Table altered.

SQL> alter table tbl2
  2    add constraint tbl2_fk1
  3      foreign key(x)
  4      references tbl1
  5  /

Table altered.

SQL> insert
  2    into tbl1(x)
  3    values(1)
  4  /

1 row created.

SQL> insert
  2    into tbl2(y)
  3    values(1)
  4  /

1 row created.

SQL> update tbl1
  2     set y = 1
  3   where x = 1
  4  /

1 row updated.

SQL> update tbl2
  2     set x = 1
  3   where y = 1
  4  /

1 row updated.

SQL>