as for Pve...i had a chance to try it ad i did quit alot, S,E & F, is very useful for 3 or more mobs, you yourself on the primary target while spirits on the other 2, they also mirror aoe abilities so u can do massive aoe damage.
Keeping one spirit active should be 20% damage burst. Am I correct?
yes you are when u summon 1 spirit you both deal 60% of your normal dmg, so 60% x2 = 120% = +20% damage boost overall, although it hurts u doing lower dmg alone, both together u do more dmg than u alone without spirit...think it matters quite a lot dont u think? Expecially with TeBrew :P
What about multi target fight? Is good to keep one spirit active at boss and damage adds
That would depend on the situation, u can spawn the spirits on adds so that your overall damage(not dps) doesn't drop, but at some point i don't think it makes any difference, tho myself..while doing dungeons using the ability on multiple targets has always increased my dps