brundlefly
Hi Bit, I hate to contradict a Grand Master, but...
 
I think the previous posts are on track. In a world where every synth has it's own dedicated virtual MIDI port and is listening on all channels, channel assignments only really matter with multi-timbral synths, and could not explain/prevent hearing two synths at the same time unless/if both MIDI tracks are echoing as described. Also, it's not possible to select None as the input if Always Echo Current MIDI Track is enabled in preferences; SONAR will automatically reset it to All Inputs - Omni.
 
OP just needs to disable forced Input Echo on the track(s), and let Always Echo Current do its job.

You know Dave, I'm gonna back up (Grand Master) Dave on this idea simply because it is actually more efficient.  Now hear me out...  In a more sophisticated set up, only using one keyboard controller, having the MIDI channels dedicated to separate tracks makes sense (even when not using a multi-timbral synth but more than one synth like the OP's scenario).  Actually it's not that sophisticated, just changing the controller's MIDI channel out when you want to control a different synth track as you play is much easier than clicking input echo off on one track and on for another.  That way it's on the fly, unless making a channel change on an Axiom is not easily accessible.   Do you get where I am coming from?  But, sure, turning echo on and off will work as described.  It's just not they way some of us would set things up like Dave or me (not that I am in the same class as Dave).
 
Kind regards,
 
Living Room Rocker

Sorry, Bill.  I guess I missed your post.  At least G.M. Dave and I are not alone on the idea.
 
Kind regards,
 
Living Room Rocker

I'll weigh in here as well and in true diplomat fashion back up both sides.
 
While the channel idea is perfectly workable, when using softsynths in Sonar, the input echo button still needs to be turned on. For example I don't use the "Always echo current MIDI track" option so specifically have to turn input echo on a soft synth track on else I don't hear anything.
 
Where the channel idea falls down is if a synth only listens to one channel, which IIRC correctly Dim Pro does. I can't check at the moment but I'm fairly sure I'm correct in saying it only responds to channel 1 so in this situation the input echo button is a safer bet.
 
For the OP. This video explains MIDI routing, set up etc

I usually turn off the option that activate track always is echo enabled.
 
If you manually turn on echo on a track and switch track such confusion may arise - getting midi played to anwanted synths.

FastBikerBoy
Where the channel idea falls down is if a synth only listens to one channel, which IIRC correctly Dim Pro does.

 
I had to double-check, but no, Dim Pro responds on all channels unless you have it in multi-timbral mode where each Element gets it's own channel by default IIRC ().
 
Right offhand I can't think if a single "mono-timbral" synth that cares what channel it receives by default, although some allow you to restrict them.
 
Regarding use of Always Echo and specific channel outputs from your controller, I use whatever combination of settings suits a particular project/situation. Sometimes I need the control, but often I don't. If a project is all soft synths with no layering, recording everything as Channel 1 and using Always Echo is very fast and convenient.
 
Coming from the hardware synth world, and using mostly multi-timbral synths often daisy-chained on the same port, I used to be a real stickler for keeping the channels of recorded events unique for different instruments,  but I find that it seldom matters any more, and I just don't bother. If a need arises to differentiate channels after the fact, and setting a forced MIDI output channel on the track isn't enough for whatever reason, it's easy enough to use the Event Inspector to quickly change the channel of selected events/clips/tracks.
 
Ports are the new channels.
 

lfm
I usually turn off the option that activate track always is echo enabled.
 
If you manually turn on echo on a track and switch track such confusion may arise - getting midi played to anwanted synths.

That's what happened to the OP, so we come full circle.