Perhaps an example will help illustrate the difference between 2-cycle DPS and normal DPS.
You are a caster with 1000 mana. The next fight is 90 seconds long. You have the following two spells:
Burn Spell - 1.5s cast, 500 damage, 100 mana cost
Efficient Spell - 1.5s cast, 100 damage, 10 mana cost
What combination of spells produces the maximum damage?
Rather than actually using an algorithm, we'll just list out all the possibilities:
The most damage comes when you cast 4 Burn spells and 56 Efficient spells. Now let's make a slight change, and add 20 mana to your mana pool, for a total of 1020 mana.
This time, maximum damage comes from casting 5 Burn spells and only 52 Efficient spells. You even spend 4.5s doing nothing! The extra mana, a Longevity stat, allows you to do more damage by spending more time in burn mode. Note that this system is also very sensitive to fight length. If the fight was longer or shorter, that would have produced a different result. You could also increase damage by adding DPS stats, making each individual spell hit a bit harder.
Now, contrast this system to the process used by my Destruction (Shadow) Warlock. I don't have two spells, I only have Shadow Bolt. Adding 20 mana doesn't really do a whole lot. I spend all of my time and effort making my Shadow Bolt hit harder (spell damage, crit rating) and more often (hit rating, haste). Longevity stats are far less useful than DPS stats.
Of course, optimizing those DPS stats is pretty complex. But it's a completely different process than balancing Longevity and DPS stats.