CREATE TABLE T1 (
  ID INTEGER NOT NULL,
  FK INTEGER NOT NULL
);
CREATE TABLE T2 (
  ID INTEGER NOT NULL,
  FK INTEGER NOT NULL
);
ALTER TABLE T1 ADD CONSTRAINT PK_T1 PRIMARY KEY (ID);
ALTER TABLE T2 ADD CONSTRAINT PK_T2 PRIMARY KEY (ID);
ALTER TABLE T1 ADD CONSTRAINT FK_T1 FOREIGN KEY (FK) REFERENCES T2 (ID);
ALTER TABLE T2 ADD CONSTRAINT FK_T2 FOREIGN KEY (FK) REFERENCES T1 (ID); 

CREATE TABLE TEST (
    F_FROM  INTEGER NOT NULL,
    F_TO    INTEGER NOT NULL
);
ALTER TABLE TEST ADD CONSTRAINT TEST$PK PRIMARY KEY (F_FROM, F_TO);
ALTER TABLE TEST ADD CONSTRAINT TEST_TEST FOREIGN KEY (F_TO, F_FROM) REFERENCES TEST (F_FROM, F_TO) ON DELETE CASCADE;

execute block as
begin
  insert into test (f_from, f_to) values ( 1 ,  1 );
  insert into test (f_from, f_to) values ( 2 ,  1 );
end

execute block as
begin
  insert into test (f_from, f_to) values ( 1 ,  2 );
  insert into test (f_from, f_to) values ( 2 ,  1 );
end