When I dual cast Summon Atronach or Raise Dead, does this simply increase the duration or does it increase their power?

Duration

You'll find that the permanent versions (thralls) require two handed casting.

It increases the sheer awesomeness of the spell.

Wait, no. Duration.

So dual casting conjuration is a wasted perk, then?

Yup. It isnt even useful for Bound Weapons...

Is it just me who finds it hellish hard to dual cast anything on 360?, I press the triggers at the same time but half the time I just end up with the spell going off in each hand, annoying as hell trying to mess about getting it working in the heat of battle.

Silly waste of magic anyway, why is it so expensive to cast when it's only a small improvement over normal casting?.

It does make the Bound Bow last five minutes, if you can afford the cost to draw it mind you.