The final perks have been released and they do refer specifically to Atronachs, so it would seem tht they're the only ones you can summon two of. However we already know about the ghost creatures and in the streams necromancers have been seen summoning corporeal undead as well, and at PAX I believe it was Matt Carofano who confirmed you can summon Dremora.
I think it's a bit lame that you're limited to atronachs for two at once. Should be easy to mod that so you can get two Daedra/undead in any combination
It does sound pretty lame. Two Atros could be Flame, Storm or Frost, I assume. Still, It shouldn't be so limited.
I'm confused on the wording, "Twin Souls" - You can have two atronachs or reanimated zombies.
Does that mean it's only two of one type?
Can you have one Atro and one reanimated zombie?
Does reanimated zombie mean ANY reanimated creature, chicken or NPC?
Does reanimated zombie mean ANY other summoned creature?