It could be me, but I really don't get what actually is your problem.
The STS 24/42 has 24 channels.
Combining mixers give you as much channels or busses as you want.
Different keyboards for different parts? hm, no idea what you mean.
But I give up
I think I understand what deBoer means.
In audio post-production for film you can have many tracks and you must group them in many stem(group) for recording them in a multitrack recorder, one for voices, one for foley, one for background and one for music.
If you work in 5.1 format you need a 5.1 pan for each mono track.
All stems(groups) are 5.1 so you need 6 channel for each stem.
In the mixer you must have the possibility to assign a track to any stem(group)you want .
All stem are in 5.1 format so there is 6 channel for each stem
4*6=24 you need 24 channel for these stem.
What would be beautiful is a STM48S with 4 assignable stem(group) in5.1 and direct output (24)for these stem for recording and 6 output for discret monitoring after summing all stem.
Can someone make this incredible mixer?
PS: you can't use more than one mixer....imagine you want to listen one track...you have solo one track and then mute all other tracks on the other mixers...that's impossible.
sorry, I forgot about the surround setting of Bear.
But I don't think there's no workaround to find.
What about recording in several stages, to name the obvious?