Hi @deadf1sh . It is knowned that the homebrew app can only import officials songs.
As for doing local multiplayer, first of all region locks could still apply. Second, are you doing download play (where only one copy is use) or each 3DS has its own copy? Did you make sure all 3DS have the latest game update? I haven't try local multiplayer but I can exchange custom songs between two 3DS.
As for doing local multiplayer, first of all region locks could still apply. Second, are you doing download play (where only one copy is use) or each 3DS has its own copy? Did you make sure all 3DS have the latest game update? I haven't try local multiplayer but I can exchange custom songs between two 3DS.