WiiPower claims to have multi-disc games working, perhaps nobody has tested this game yet. Did you read the NeoGamma readme and other text documents I included in the download, there should be something in there reguarding multi-disc games.
Here is some info you may find interesting
Put several GC games on a single game disc:
MultiGame ISO Creator v3.5.4F+
MultiGame ISO Creator v3.5.4F+.zip
(Attention: Please use "fstfix t" on all shrunken .isos, after shrinking, before putting on multi game discs. This should fix all issues with games that "don't work shrunken". And it might also increase the reading speed for those games.) The reason for this is that audio streams(.adp files) need 32KB alignment, but the shrinking kills it, "fstfix t" gets all files to 32KB alignment.
"Fix" GC .iso files
Wiped GCM Fixer 1.5F also callled Fstfix 1.5F
FileTrip - fstfix.exe 1.5f
Baten Kaitos works if both discs are put on the same multi game disc with MultiGame ISO Creator v3.5.4F+, and the 1st disc is patched with the options:
FSTFIX BKdisc1.gcm t s g
and the 2nd with these:
FSTFIX BKdisc2.gcm t s f