inspired by the “Dual Voice” button on my 1990 Yamaha PRS-38 arranger keyboard / sortofish MIDI controller. where the button merely causes notes played / sent to the MIDI IN to be simultaneously voiced by whatever the second to last Voice parameter to be selected was in addition to the normally expected behavior of all keyboards of just playing the most recently selected Voice.
if they did it with a 1990 MIDI keyboard then this bare bones basic implementation should be doable on the BB, with a Settings menu option being the toggle for Dual Voice ON/OFF.
could even do more than two voice polyphony of course, but i would imagine that more than two is pretty likely to get just more messy than cool. more advanced functionality like setting relative volume level of each voice would be nice but completely not necessary when we can do that stuff ourself with BBM as intermediary if we want. no need to waste time doing more than just bare bones implementation for this feature.