Implement pseudo 8-operators (a.k.a. dual-voice) mode

[Wohlstand]

This mode often used in various games with playing same notes in two channels

[freq-mod]

Maybe not necessarily 8-OP mode, but dual-voice/dual-channel mode would be a great addition.
Under "Shut up!" button there is some space, so there could be "enable dual voice channel" checkbox, fine tuning bar, second voice volume bar, note offset bar and that pretty much would be enough.

It would MASSIVELY help with instruments such as Synth Brasses, Pads, melodic FXs and what's not, as I'm constantly struggling to make any one of them sound reasonable and in most cases it is fail

[Wohlstand]

The "Pseudo-8op" is directly means "Dual-voice" mode which is same as on DMX drivers are had "Pseudo-4op" which is same.