If I understood you correctly, you need to know if a record exists in both tables based on the ID and FK_ID fields. If that's true, you can use a simple join query like
SELECT COUNT(*)
FROM Table1 a
INNER JOIN Table2 b ON a.ID = b.FK_ID
WHERE a.FirstName = @FirstName
AND a.LastName = @LastName
AND b.Mail = @Mail
AND b.Phone = @Phone
That should the amount of rows that are matching and zero if none.