Вы можете присоединиться к таблице STATES.Что-то вроде этого:
SELECT X.STATE, X.NUMBER1/CAST(Y.NUMBER2 as FLOAT) as PERCENTAGE
FROM
(
SELECT S.STATE, COUNT(*) as NUMBER1
FROM SOMETABLE1 AS S1
INNER JOIN STATES AS S ON S1.LOCATION = S.STATE
AND S1.DATE >='10/1/2008'
AND S1.DATE < '10/1/2009'
GROUP BY S.STATE
)X,
(
SELECT S.STATE, COUNT (*) as NUMBER2
FROM SOMETABLE2 AS S2
INNER JOIN STATES AS S ON S2.LOCATION = S.STATE
AND S2.DATE >='10/1/2009'
AND S2.DATE < '10/1/2010'
GROUP BY S.STATE
)Y
WHERE X.STATE = Y.STATE