Is it an increase in summon's duration, summon's power, or both? The spell description is wonderfully vague.

If anyone has precise numbers for the increase, even better.

I'd assume both but I'd like to know as well.

No one has this figured this out yet, eh?

The shortcut / dual-wield mechanisms being the huge train wreck they are, you can "dual cast" conjuration spells without even having the perk - I have the one for destruction, and when using two destruction spells, tapping a conjuration shortcut twice equips the spell in both hands.

No idea about the power or duration but it does cost twice the mana.

Dual-casting perks combines both hands into one spell, making it more powerful. Healing, for instance is faster with the combo spell than with two hands healing at the same time.

That's different from the dual casting perk. You can do that with any spell, but you're just casting the same thing twice, rather than combining two spells into a more powerful single spell.

I do know that it increases duration. By a lot.

I don't know if it increases strength, but I'd imagine it does.

So there's no point in dual casting a fire spell, since flames + firebolt is noticeably better than any of the two combined with itself ?

This, it doubles the duration.