Here's how I handle it. Not by the books, but this isn't the Rules Forum, so it should be fine.
I calculate experience for each character seperately based on their level as if the entire party were composed of
n level characters.
Example:
Using the chart from the DMG, and the group you listed.
11th level character
10th level character
9th level character
7th level character x2
6th level character
.. 6 characters, overcoming a CR 9 encounter.
The 11th level character receives 275 (1650/6) XP.
The 10th level character receives 333 (2000/6) XP.
The 9th level character receives 450 (2700/6) XP.
The 7th level characters receive 700 (4200/6) XP each.
The 6th level character receives 900 (5400/6) XP.
... the same 6 characters, overcoming a CR 11 encounter.
The 11th level character receives 550 (3300/6) XP.
The 10th level character receives 750 (4500/6) XP.
The 9th level character receives 900 (5400/6) XP.
The 7th level characters receive 1400 (8400/6) XP each.
The 6th level character receives 1800 (10800/6) XP.
The 6th level character ears XP at about 3 times the rate of the 11th level character. After 40 encounters (assuming all CR 9 encounters), the 11th level character is ready for 12th. The 6th level character earns enough after 7 adventures to level up to 7th. After 10 adventures, he is now ready to be 8th. After 13 encounters, he is ready to be 9th. He'll be about halfway through 9th when the lead character levels to 12.
While this is inherently unfair to the high level characters, the high level characters are vastly more powerful and won't feel as challenged. It's a tough situation to overcome, once level differences start to exceed two or three, for the lower level characters to even make appearances in a fight geared for CRs well above them.
Greg