If MIDI OUT Mode is set to “SEQ”, only the first (or current) played MIDI note of a sequenced or arpeggiated duophonic sound is sent.
Therefore when recording the MIDI notes of a sequenced/arpeggiated duophonic patch, all you see are single note lines. And playing back those recorded tracks will naturally sound quite different from what was played.
This does not happen when MIDI OUT Mode is set to “KEYS” (but then you don’t get the sequenced/arpeggiated notes)-
Thanks Oli !
I wonder if it’s something which also happens when using firmware 1.1.0.
Yes, I observed this behavior with firmware 1.1.0 as well but I didn’t do any in-depth tests as with 1.1.20.
I remember this was on my list to implement post-v1.1.0… It’s possible I still haven’t added it. I’ll take a quick look.
I know there’s one improvement to sequencer MIDI output in the latest firmware; now tied notes of the same pitch are output as single longer MIDI notes, where previously they were output as multiple overlapping MIDI notes, which doesn’t work for multi-triggered sounds.
This is still true for the arpeggiator… the MIDI output doesn’t accurately reproduce what’s happening when you are actively holding notes on the keyboard, when MIDI OUT mode = SEQ.
For now if what you want is to record MIDI and get duophonic arp action when you play back the MIDI, best way to do it is to (possibly turn off Local Control and) set MIDI OUT mode = KEYS… record the actual key-presses that generate the arp… and leave the arp ON when you play back the MIDI. Tested that and I get the correct behavior on playback.
The sequencer meanwhile really is putting out two duo notes per step as MIDI notes, if you record while in Duo Mode and enter two pitches on a given step… if on the other hand you record only one pitch per step within the sequence and are holding down different duo notes live while you play… this is a scenario similar to the arpeggiator above. You can get the desired audio on playback by recording the MIDI of the keypresses (both the root note for running the sequencer and the note you’d be holding for the duo part) and playing this back to the Sub37 while its sequencer is on, with Seq Options-> MIDI OUT set to KEYS. This setting is necessary in order to have incoming MIDI trigger the arp/sequencer, when Local Control is off.
hope this helps… I think a little tutorial supplement is probably in order, summarizing all of this stuff. 
Ok, this is exactly where I got it wrong. In my sequences, there’s only Pitch 1 programmed and the other voice is achieved by pressing the keyboard. I just tried, and indeed, when both pitches are defined in the sequenced, they are played as well. It would be great if there was a way to influence the programmed Pitch 2 by playing a second note on the keyboard. That would be perfect, IMO. Here’s the reason why: If you want to play duophonic sequences live, you could program the octave/rythm pattern for the lower and upper notes, and then play the desired pitches on the keyboard. That would open up a new dimension of what can be played live (at least for me 
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This would be much appreciated.
BTW, please have a look at my other inputs regarding the Sequencer Oddities as well.