Skywrath Mage has a heavy damage ultimate, and after reaching level 16 its damage is further increased to 1400. If Aganhim's Scepter is purchased for him, his ultimate's cooldown reduces to 0. With this in mind, if 2 ultimates are used on the same hero, will the ultimates stack?
Yes, it stacks. However you will need a huge manapool (or an Obsidian Destroyer) because this built is really mana intensive.
You can see here that DPS does stack up.
But in practical games it doesn't end up so effective,unless you are snowballing crazily and have some serious disable in your team.
It's just too hard to farm up items that give you enough mana to pull this off,and once you do enemies will be too tanky,will kill you too fast or won't be disabled for long enough.