I've never tried this skill (except on a tumbletrack), and I've never trained it, but I imagine all of the drills that apply to backhandspring double-backs should apply to whip double-backs as well.
As I see it, there are two approaches to learning double-backs; one for gyms where you can tumble into a pit, and one for gyms where you can't.
If you can pit tumble, I'd do it there. Once you can do it comfortably into a pit with time to spare (and make sure your awareness is good enough that you can open up and spot the landing), do it onto a mat (same, make sure you can spot the landing), then do it on floor.
If you can't pit tumble, do a back tuck onto your back on a high stack of mats until you feel confident enough to try it on floor (preferably with as soft a landing as you can arrange to have.)
You want to land the whip with your feet behind you; in other words, the whip should probably be lower than it would usually be. This is best accomplished by making sure you have a good hard snap down on the roundoff, giving you a fast, low, powerful whip.
But as I said, this is all going off how I would teach it; I have zero experience actually doing so.
EDIT: Not to be discouraging or anything, but I want to make sure you're aware: there would be no connection bonus for a whip double-tuck.