Вы должны определить перед триггером для удаления. Попробуйте использовать два триггера
CREATE OR REPLACE TRIGGER INS_TABLE1_NUM_TRG
AFTER INSERT ON table1
FOR EACH ROW
BEGIN
UPDATE table2
SET table2num = :new.num
WHERE table2.id = :new.table2_id;
END INS_TABLE1_NUM_TRG;
CREATE OR REPLACE TRIGGER DEL_TABLE1_NUM_TRG
BEFORE DELETE ON table1
FOR EACH ROW
BEGIN
UPDATE table2
SET table2num = (SELECT num FROM
(SELECT num FROM table1 WHERE table2_id = :old.table2_id
ORDER BY modification_date DESC)
WHERE ROWNUM <= 1)
WHERE table2.id = :old.table2_id;
END DEL_TABLE1_NUM_TRG;