They don't stack at all.
Not that they are worth having her out.. Maybe in an expansion or two when people can acquire vast amounts of spell/skill speed stats on their gear, but for today's gear lvls? No, keep Selene out.
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You might be right, I don't use her tbh.. I don't find her useful. She didn't even get any of the spotlight during sch quests.
edit: Yep you were right, they don't, dunno why I thought they did, maybe I had hoped they did? Don't know but even less reason for her to exist in my book. I'll edit my previous post and remove it so that misinformation doesn't spread.
I like to use Selene for soloing and Eos for Dungeons. I do this, because I have to worry about more people than just myself in a dungeon... Plus... I don't want people having a go at me for apparently not healing them...
right now Selene is really bad, i would change that cast speed to maybe a 10% dmg buff instead.
My FC really likes when I use my Selene faerie on Garuda and other fights we have on "farm". The 30% spell casting speed and the 30% skill speed are a huge increase in DPS for the whole raid. I also use this in AK because there is no need for Eos's tool kit. I can heal through anything while DPSing. If you're learning a fight Eos's is great for the extra AoE heals and damage mitigation. Once you've geared up a bit and are confident I recommend giving Selene a try.