I was not aware that there is a 6 month self life on DDX. I have used DDX many months after it was opened and there is no apparent difference in the look or action of the developer. It is probably not as long lived as HC-110, but it keeps pretty well for me.
Ilford Multigrade developer for paper is on the other hand a very poor keeper and turns rather quickly in concentrate. I buy 1/2 gal. quantities and decant it into smaller bottles and then it keeps well.