MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE test1 (t1_id INT PRIMARY KEY, t2_id INT);
CREATE TABLE test2 (t1_id INT, t2_id INT PRIMARY KEY);
ALTER TABLE test1 ADD FOREIGN KEY (t2_id) REFERENCES test2 (t2_id);
ALTER TABLE test2 ADD FOREIGN KEY (t1_id) REFERENCES test1 (t1_id);

Records: 0  Duplicates: 0  Warnings: 0

Records: 0  Duplicates: 0  Warnings: 0

INSERT INTO test1 VALUES (1,11);

Cannot add or update a child row: a foreign key constraint fails (`fiddle`.`test1`, CONSTRAINT `test1_ibfk_1` FOREIGN KEY (`t2_id`) REFERENCES `test2` (`t2_id`))

INSERT INTO test2 VALUES (1,11);

Cannot add or update a child row: a foreign key constraint fails (`fiddle`.`test2`, CONSTRAINT `test2_ibfk_1` FOREIGN KEY (`t1_id`) REFERENCES `test1` (`t1_id`))

INSERT INTO test1 VALUES (1,NULL);
INSERT INTO test2 VALUES (1,11);
UPDATE test1 SET t2_id = 11 WHERE t1_id = 1;
SELECT * FROM test1; 
SELECT * FROM test2;

Rows matched: 1  Changed: 1  Warnings: 0

t1_id
t2_id

1
11

t1_id
t2_id

1
11