MariaDB: Insert records (list of tuples) into table if not exists
Your SELECT with two select can be JOINed or UNIONìted
As you want to insert two rows, you need UNION like this
INSERT INTO `test` (col1, col2)
SELECT * FROM
((SELECT * FROM (SELECT '1','2') AS tmp
WHERE NOT EXISTS (
    SELECT * FROM `test`
    WHERE col1='1' AND col2='2'
) LIMIT 1)
UNION
(SELECT * FROM (SELECT '3','4') AS tmp
WHERE NOT EXISTS (
    SELECT * FROM `test`
    WHERE col1='3' AND col2='4'
) LIMIT 1)) t1;

I don't see why it should give you an error
INSERT INTO `test` (col1, col2)
SELECT * FROM
((SELECT * FROM (SELECT '1','2') AS tmp
WHERE NOT EXISTS (
    SELECT * FROM `test`
    WHERE col1='1' AND col2='2'
) LIMIT 1)
UNION
(SELECT * FROM (SELECT '3','4') AS tmp
WHERE NOT EXISTS (
    SELECT * FROM `test`
    WHERE col1='3' AND col2='4'
) LIMIT 1) 
UNION
(SELECT * FROM (SELECT '9','9') AS tmp  WHERE col1='9' AND col2='9')
) t1;