i have 2 characters now, and i was wondering if lets say on 1 of them i learn 10 shouts, then on the other i learn 10 shouts, do they then unlock the 20 shout acheivement??
thanks alot in advance!

anyone knows?

One assumes that that particular achievement requires 20 different shouts.

I'd guess that it doesn't add up to 20 and unlock the achievement. You'd have to hit 20 with one character. Achievements unlocked will stay unlocked though. I unlocked one then loaded my game from an earlier point and I still had the achievement... this is probably because the achievements are through Steam.