your not stopping on a different gear because the cranks are part of the same shaft. If the 90-11 stops moving then the 20-20 also does. It would be the
(90-11 + 20-20) /2 ....I'm guessing, but it definatly would not be different for one side of the crank than the other.

[edit] I'm trying to think free wheel but keep thinking dual fixed. Maybe you are right but the momentum on the 90-11 is far greater than a momentum on the 20-20