The SQL statement would look something like

SELECT TOP 2 DueDate
FROM YourTable
WHERE DueDate >=Date()
ORDER BY DueDate

If you can only use the query grid
Field: DueDate
Sort: Ascending
Criteria: DueDate >= Date()

Then set the query's Top Values property to 2

If you table contains multiples of the same date then you will need to build
a distinct query first and then use that in place of  the table.  You might
be able to do that in a single query that looks like the following in SQL.

SELECT Top 2 DueDate
FROM
 (SELECT DISTINCT DueDate
  FROM YourTable
  WHERE DueDate >= Date()) as UniqueDates
ORDER BY DueDate