Ooh that's small, I'd wouldn't run anything less than the 10 ltr option.
Yes double di will help out a great deal,
My membrane is on its last legs and is at 45 TDS at present, i double di the the first bottle at 10 and new resin in the second (11 ltr) I get around 8000 ltrs before the second bottle hits 10 TDS ( that's about 4 days work )
Must get a new membrane next week

Darran