I’m thinking the latter.The specific output (deck and channel routing) would be already set in the main Djay program settings.
Assuming the mapping of the Djay decks to USB Channels of the interface/mixer are how you want them then everything else can be done using MIDI commands.
Something along the lines of:
(Left to right in 4 decks mode)
Deck 3, Deck 1, Deck 2, Deck 4
Button X pressed =
Load selected track on Deck 1, 3, 4
Set Deck 3 to Vocals
Set Deck 1 to Drums
Set Deck 4 to Harmonics
Button Y pressed =
Load selected track on Deck 2, 3, 4
Set Deck 3 to Vocals
Set Deck 2 to Drums
Set Deck 4 to Harmonics
Crossfader between Decks 1 & 2 to allow mix between tracks. Allow channel-based separation of Stems on the “live” track but inbound mix on the opposite deck 1/2.
I’m spitballing at the moment, but I guess it depends on the workflow you want. But it shouldn’t be hard to get your vocals at the least loaded into another deck/channel and synced with your main track so you can monitor the vocals through IEMs
