@Leto, According to UESP, Conjure Seeker, Ash guardian, Ash spawn, they do not benefit from the twin souls perk. And an info on elemental potency: The 50% is not entirely true. Check Conjuration spells (google it and select UESP link), I cannot provide link, I'm at work and Mcafee blocks it :D there you can check the conjured atronachs' stats. An example: Frost atronach without the perk: Life 300, stamina 125. With the perk: Life 503, stamina 292.
@Ben C: thanks for your info, I'm checking now you Cursed Prophet :)