Pretty awesome encounter earlier. Turns out that if you wait for the cooldown, you can have two dragon summons at once! Here are a few pics I snapped.

All four: http://i.imgur.com/H33XB.jpg

And some additional pics in glorious iPod quality! Couldn't get all four at once in these, though.
http://i.imgur.com/npVmV.jpg
http://i.imgur.com/XdvLo.jpg
http://i.imgur.com/e33rp.png

sweet dragon showdown...i found a glitch that does away with the recharge meter and had both dragons and a storm at once

how did you get rid of the cooldown?

I'd like to know as well. I just had to derp around for a bit while Odahviing attacked, waiting to summon Durnehviir.

Essentially, you do the restoration potion glitch, drink the overpowered restoration potion, then put on an Amulet of Talos right after. The effect will stay until you take the amulet off.

Just to clarify - as I've never used that glitch - fortifying restoration buffs your enchanting? Or something like that?

yep thats it...the fortify restoration potion has to be a really high value and once you drink it you can see the recharge times on each shout word will be negative numbers

It buffs every effect on every enchanted item.

The 20% on the amulet stays the same though, so you have to check the actual shout cool times in your magic menu to see when you have gotten to 0 cool down.

^^^What Haggar said

So let me get this straight:

A zombie dragon, undead skeleton minions of said dragon, a red dragon, and Odahviing [censored] a blood and ancient dragon?

Sounds awesome to me XD

This happened to me too in winterhold. I was battling twin Ancient Dragons who didn't really want to land so I called Odahviing, but the battle dragged on so long I was able to summon Durn too.