This is NOT a repost, as far as i know. In order to do this you need to have the fingers of frost talent.
When fingers of frost procs, u get 2 frost spells as if the target was frozen, say i had it proc, then casted frostbolt, then while im casting frostbolt for the SECOND time, spam click deep freeze at the same time and not only will deep freeze work but also your frostbolt that you casted at the same time will also proc!