The chances are slim, but still possible. I'm not sure what their scheduling is for the rest of the year, nor that of their rivals. I'm not even sure if all of their competitors are still active doubles partnerships who would be hoping for a place at the tour finals.
This is how the doubles race looked on Monday. Jamie and John will be able to add 250 points, but not sure about those around them.http://www.atpworldtour.com/Rankings/Teams.aspx
Fleming and Marray are a bit lower down, but a couple of good runs could put them within touching distance.
Considering the problems they face entering the Masters Series tournaments, they've done really well, and it doesn't seem fair to have them competing against guys who are close in doubles rankings, but can enter any tournament they fancy.