i have run several 10-14 person groups for dnd and shadowrun, as long as your willing to trust a co dm ( or two in my case ) then go a head.
heres how i structured the sessions.
we usualy played for 6-8 hours total at a time, think weekends or fridays with midnight food runs.
before each session i would sit down with my fellow dm's and give them a general out line of what was going to happen, the thing about having multiple groups so to speak is that you get to be really creative with problems and stories.
since we usualy broke into three groups the roles more or less fell into the support group who runs a mission that makes the other two easier in some fashion, the "assult" crew who lead the main charge so to speak, and the "distraction" crew who well hit the beehive with a stick and ran for all they were worth ( quite literaly once with an insect infestation in a shadow run campagin ).
another way to strucure it in a more investigation way would be ok you guys go check out the creepy mansion, you guys rundown the criminal element and we will go try and track the trail of the killer.
i went ahead and planned out all of the bad guys and areas, maps, charts ect and then would give my fellow dms a flow chart and a folder with what i hoped would be everything they needed.
every few hours of real life time we would take breaks and hit up a food stand or make a run to the the truck stop, during that time i would talk to my dms and see what was going on and where in the story they were, the general rule of thumb was that if one group gets done early the dm lets the others know and then the players can either start on a new story line or wait for the rest of the group to finish up,
if one group is taking longer than the others, aka they hit a snag, smoe one got captured then you can play on that and have one group stage a rescue for another or be the gandalf on the third at first light.
basicly it all comes down to be creative and make sure your players are ready to deal with down time every once and a while, use this for book keeping or for roleplaying.
also it can be fun to bring thw whole group together for a massive encounter, this lets you do things like fight a dragon, no way the average group could pull it off, but what about a group of 10 or 12?
as for ballancing encounters for larger groups there is a trick i use for this, i call it the red dragon chalenge.
start the group off at the begining of each challenge at either full normal stats or at 3/4 power ( has some hp missing/stresses or concequences ) then start out with a weak oppenet and work your way up until your party can no longer fight the beastie off.
this lets you see several things: how well do they work together? who will step up and lead the group? what can they deal with?
when doing this encounter dont do it on an empty expanse do it in a place where a fight might really happen in a wherhouse in a alley some where with things the players can use, trees for cover, rafters for hiding in ect..
in the end though the biggest thign you can do is to keep things moving, dont get bogged down because if you do every one who isnt involved in the problem will be bored.