What a timely post!
I am on the verge of headling "head long" in to this myself over the next few weeks!
I have a track that I wanted an authentic "soul" brass section section and the virtual instruments I have are absolutely awful for this type of brass
After MUCH research I narrowed down my choices to NI Session Horns and Chris Hein Horns Compact
To my ears the Chris Hein Horns are absolutey AWESOME! and even though they are more expensive than Session Horns (£145 as opposed to £99) the sound and options are just incredible
Session Horns did have a nice feature where it would automatically split chords across the different horns
However, as with all automatic processes it is not 100% so doing it manually is probably the way to go
You are definitely on the right lines with your splits
The other keys to authentic horns are…
Make sure your MIDI notes are not perfectly lined up – it is the subtle timing differences that give that authentic “feel”
During held notes/chords spend some time adding volume automation to provide “swells”, crescendos etc.
If you have access to “articulations” make sure you throw some in – eg rips, falls etc.
You could also experiment with micro tuning adjustments, which may help to “thicken” the sound and add to the realism
I would also be very wary of adding too many horns (depending on the style you are working on) – in soul, funk etc. the horn section is usually quite small
You might be “fooling” yourself into thinking that adding more instruments is improving the sound – it might just be the volume increase making it sound "better"
You may be better off spending your time tweaking the sounds
Please post any other things you discover and I would love to hear how you get on and listen to your track
What “horns” instrument are you using?