I’m not the developer, but I definitely thought this was worth sharing here, and I’m looking forward to using it!
Polythemus can filter incoming midi notes and spread them across different MIDI channels, similar to MPE, allowing for polyphonic playing of Digitakt, Octatrack, the Nord Drums, any multimbral synth, or even allowing a chord to be split across several hardware monosynths. There have been some other great solutions for this (BomeBox, Max for Live devices), but this seems to be the first solid option for iOS.
The basic idea: for now, you’ll need the app AUM as a host, and to set up the routing. On your Digitakt, set channels 1-4 to the same sound. Tell Polythemus you want to set voices 1-4 to output to the corresponding MIDI channels 1-4. Set the voice allocation mode to Round Robin. This way, each MIDI note fed into Polythemus can output to the next available channel on the Digitakt (using AUM’s midi matrix). Chords! Notes that don’t cut each other off immediately! For more interesting results, add more or less variation to each Digitakt sound, or hell, have them play completely different sounds. Or completely different synths. Plays well with iOS Audio Units.
https://forum.audiob.us/discussion/31589/video-demo-of-polythemus-au-midi-app
I’ll update this post with an App Store link when it’s available. It has been submitted for review.