Talvez isso funcione, mas eu nunca tentei:
SELECT *
FROM T1 JOIN T2
ON T1.SOMECOL = T2.SOMECOL OR (T1.SOMECOL IS NULL AND T2.SOMECOL IS NULL)
SELECT *
FROM T1 JOIN T2
ON T1.SOMECOL = T2.SOMECOL OR (T1.SOMECOL IS NULL AND T2.SOMECOL IS NULL)
