Sven's ultimate gives you 100%/150%/200% bonus damage.
Only increases base damage and that given by the primary attribute of Sven. Raw bonus damage is not increased.
– Sven - Dota 2 Wiki
Your damage consists of 2 numbers:
A +B, and this description basically means that the ultimate gives you 100%/150%/200% of the
A number. It excludes raw damage items (but items that give Strength still count)
Exactly the same can be said about the Double Damage rune: it gives you 100% of
Since both of these only increase
B and are based only on
A, their bonuses will not be multiplied with each other; they're completely independent. But they do stack in a sense. The order of activating them does not make a difference, and it probably wouldn't make a difference even if they stacked in some complicated way.
So if you have both Double Damage and Sven's ultimate level 3 active, your total damage will be:
A+B + A*2 + A (the
2 stands for 200%).