I seem to have no problem using a command such as:
CODE
metaflac --set-vc-field "ARTIST=John Doe" song.flac
Repeating that command with a different name
CODE
metaflac --set-vc-field "ARTIST=Jane Doe" Song.flac
adds that tag to the comments. Then doing a metaflac --list shows:
CODE
METADATA block #2
type: 4 (VORBIS_COMMENT)
is last: false
length: 170
vendor string: reference libFLAC 1.1.0 20030126
comments: 8
comment[0]: TITLE=Song
comment[1]: ARTIST=John Doe
comment[2]: ALBUM=Witness Protection
comment[3]: TRACKNUMBER=1
comment[4]: DATE=2004
comment[5]: GENRE=Rock
comment[6]: ARTIST=Jane Doe
I have no idea how various tag readers or transcoders will handle this, but I do believe that this is the way multiple artists were intended to be dealt with in FLAC tags.