Group the notes mentally into twos and break the scale down into two sections, 1. right hand on black notes and left on white, and 2. right hand on white notes and left on black. Which comes first depends, obviously, on the starting note. There will be one remaining pair of notes where the group of two will be white+white; this is the point where your hands will change position. Add this pair onto the group which precedes it. Now practice these two groups separately with relaxed wrists, tension will kill the speed. Start andante or possibly slower and play each group rhythmically and evenly, repeating over and over, progressively speeding up - ultimately 12 notes per second should be attainable (i.e. the full gesture will last as many seconds as it is octaves). Use a minimum of vertical movement, the hands should skim over the top of the keys. Once each of the two groups is secure and fast, put them together.
Whichever hand is on black notes should be further into the keyboard and at the changeover point one hand goes in and the other comes out. You can see the sort of zig-zagging of the hands clearly in the video of his Carmen fantasy. This advice should work irrespective of where the starting and finishing notes are (though it is just possible that the octaves could be written so that you'll be starting on the changeover). Normally if ascending the octaves will start on the lh; if descending with the rh. View whichever hand starts first as the leader and the other one should follow.
(edit: I should add do the intial stage without pedal, only add pedal at the final stage. Incidentally manipulation of the pedal can be used to add rhythm to the final scale, depends on what effects you want though. How much pedal you use, and whether you pedal unchanged through the whole thing is another issue altogether).
Topic: Chromatic Horowitz Octaves