I love to play brass too.
And I thought about this feature long ago, and wanted to add it on vArranger. The only problem with that feature is that you need to add some LATENCY. Because the player can't play 2 notes or 3 or 4 exactly together.

So it need to add a rule.

When it get the first note, it wait for some milliseconds, to see if new notes is coming together.

If no, it plays every channels (trombone, trumpet, sax, brass)

If yes, then it then assign every note, to an instrument.

So, how much latency you want to add to your playing?

Ketron keyboards have a function called DUET, TRIO who allow that too, but it's not perfect.