While browsing the wiki I found an alternate local multiplayer mode called "mugen ensemble" that is not region lock so it works even on EU and US console:
you need to hold L button while selecting local muti. it then asks if you're leader or member.
It's not really good because:
-you both need to possess the same song
-you only hear your instrument.
All it does is that it sync the song between two (or more?) 3DS.
Some other tips (if i google translate right ):
you can press any A.B.X.Y instead of shooting or blowing in the microphone before a concert.
you can obtain tomato with the "scout" feature at the end of a concert (i'm really not sure about this one nor how it works..)
you need to hold L button while selecting local muti. it then asks if you're leader or member.
It's not really good because:
-you both need to possess the same song
-you only hear your instrument.
All it does is that it sync the song between two (or more?) 3DS.
Some other tips (if i google translate right ):
you can press any A.B.X.Y instead of shooting or blowing in the microphone before a concert.
you can obtain tomato with the "scout" feature at the end of a concert (i'm really not sure about this one nor how it works..)