As a general rule, synths and samplers make more efficient use of resources when used in multi-timbral mode over separate instances for each instrument. This is true with both Omnisphere and Kontakt.
 
The exception would be on a resource-strapped machine where you're struggling with dropouts, in which case separate instances might serve you better because you can freeze them as you go. Omnisphere can be quite demanding, depending on the patch. On my old dual-core machine it was not practical to use multis most of the time. But if that's not your situation, then I'd recommend the approach you're already taking.
 
I use this technique often, driving multiple Multi slots with separate MIDI channels, so I can at least verify that it does work. I'd check the routing on your two MIDI tracks and make sure the second one's MIDI channel is explicitly set to "2:Omnisphere" in the dropdown list labeled "C" (for channel).

Okay this will tell us if you have an audio interface or cpu power issue. This is a link to a template I have set up with Omnisphere pre-routed to 8 midi tracks and 8 audio tracks https://1drv.ms/u/s!Ah63TBpKAVf5haQM8GYpR0TwmryO2A
If you get no sound after selecting a sound in the second channel then it may well be an audio interface issue that we will try and help you sort out. Incidentally Omnisphere can be run multi-core but you will have to set up separate instances of the VST for this to work, as each instance uses one core. For example if you have an 8 core machine you can set up 8 instances of Omnisphere and have one patch in each one.

bitflipper
As a general rule, synths and samplers make more efficient use of resources when used in multi-timbral mode over separate instances for each instrument. This is true with both Omnisphere and Kontakt.
 
The exception would be on a resource-strapped machine where you're struggling with dropouts, in which case separate instances might serve you better because you can freeze them as you go. Omnisphere can be quite demanding, depending on the patch. On my old dual-core machine it was not practical to use multis most of the time. But if that's not your situation, then I'd recommend the approach you're already taking.
 
I use this technique often, driving multiple Multi slots with separate MIDI channels, so I can at least verify that it does work. I'd check the routing on your two MIDI tracks and make sure the second one's MIDI channel is explicitly set to "2:Omnisphere" in the dropdown list labeled "C" (for channel).

I got it to work!!!
 
I had to remove all MIDI and audio channels and instances of Omnisphere and start from scratch.  Starting with an instrument track and then splitting them messes around with the MIDI.
 
Thank everyone for the help.  Going to check that MIDI buffering thing out now.
 
PS: I strongly doubt I have a CPU issue.  A look at my signature will confirm :)

KPerry
 
 
This sounds more like an incorrectly set MIDI Prepare Buffer in SONAR - try increasing of descreasing it (I always forget which direction solves this!).

I increased the MIDI buffer from the default 250 ms to 500 ms and it helped.  It seems a bit excessive considering how powerful my cpu is.  It probably also depends how much MIDI is processed.  My whole track is MIDI/VST based...

Bringing this topic back from the dead.  I have my MIDI and audio tracks set up (4 of each); MIDI input to Channels 1-4 of my MIDI Controller (M-Audio Oxygen 61) in Sonar and CH. 1 - 4 in the Omnisphere mixer. 
 
Audio Output is fine. But Omnisphere only receives data from MIDI channel 1, nothing from 2-8.
 
I want to assume this is a routing issue on the Sonar end of my system, not Omnisphere.  In my MIDI input options, I am using MIDI Channels from the Oxygen 61 sub-menu.  For everyone else, do you have a sub-menu specifically for Omnisphere ( i.e. Session Drummer 1)? Maybe I'm missing something like that on my end. 
 
Thanks for the help, all. 
 
EJ  
 
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Update:
 
Operator Error. Was changing MIDI Channel via I/O rather than through the MIDI settings.
 
EJ