Unfortunately, Song Mode doesn't work in Multiplayer. You will need to man each DS individually
However, if you are using Korg DS10+ in Dual Mode and on a compatible DSi then you can run Song Mode - with the extra mute voice functionalityso you can have 1 musician - 4 Synth voices plus 8 Drum tracks ( program the drum sounds into more-instrument sounding patches such as a bass or drones etc.
Changing between Deck A and Deck B can be very quick using the shoulder buttons - in theory - you should be able to accomplish huge works on just one DSi without using multiplayer
Hope this helps - but yes, I was a little disappointed when i discovered I couldn't use Multiplayer Song Mode. .