You're out in the desert/sunset zone 12? If it is still hot in the day time there, I'd suggest waiting a bit later into the fall. In general dividing perennials such as Coreopsis is best done before it gets too cold, but not when the divisions will stress from heat and loss of roots. I generally cut back the foliage by at least half to get them to not dry out too quickly from loss of roots. Day time temps in the 60's are probably an ideal range for quickest rerooting.