FYI
I confirm you can update the DSSTWO using your N3DS.
Mine was on version 1.1 (EU version)
1) I copied the dstwoupdate.dat to the root of my memorycard
2) started the DSTWO, which asked if I wanted to update (ofcourse I did!)
3) update succesful
4) removed the dstwo (just to be cautious) and updated the 3DS firmware (v2.0.0-2E)
5) After reboot, insert dstwo and... tadaaa... all works!
Otherwise,If you have already updated your 3DS to 2.0.0-2 , well ,
Noticeon't update in your 3DS, you need to take a DS/DSi to do the update, here is the method
QUOTE(Arschkrampi @ Jun 7 2011, 12:26 PM) Put the file from the archive in root folder of sd card.
Insert flash card in ds and start.
It'll ask you to update. Done.