Hello,
There may be 'intelligent' hubs like the MOTU that can be programmed so that certain outputs send certain channels ONLY, but the usual way to resolve this issue is to use the facility whereby most midi devices can be set so that they receive on certain channels ONLY. This facility is there for this purpose. You would need to spread the available channels between the devices to suit your overall plan, otherwise all units will play all channels that they can play on, and that they receive data on.
Of course, it might be easier to use the power switch on the playing units to control whether they actually play or not, OR, if the Audio output of the units is running into a mixer of some sort, then mute the sounds that are not required.
Geoff