Sonar X3 user. I am using both Session Drummer 3 and Dimension Pro. I must be choosing the wrong options when I first insert a soft synth. Once I have session drummer on track and happy with all beats measures sounds etc, I insert Dimension Pro to pick let's say a bass part or violin. When I play my M-Audio Axiom it now triggers Session Drummer as well as Dimension Pro? Am I not able to move on and work on another track without affecting what I already laid down?

 It sounds like you have the input echo turned on in your tracks.  It's just to the right of the record arm button.  Try turning that off.

Hi,
 
Check to make sure that the button input echo button is not on for the Drum Track when playing the DP track is not on.  I have had that to happen on occassion especially sometimes when using midi omni with M-Audio controllers. You could also set different midi channels for the two instruments. IHTH....

To my knowledge it was turned off. I am sorry if I am not explaining correctly because I am at work as usual. Its not just that I hear Session Drummer, but the fact that I am triggering it when clearly having chosen Dimension Pro track to work on. I am trying to recall the original window that pops up when inserting a soft synth and thinking I am choosing the wrong options to begin with?

The way Sonar works with soft synths, by default, it will automatically turn the midi Input Echo button 'On' when you put a midi track into focus by selecting that track.  At the same time, it also by default will turn midi Input Echo 'Off' for whatever previous track you had selected.
 
The only way that comes to mind for both to be sounding is that midi Input Echo is On for both midi tracks, and that is telling Sonar to produce sound for both soft synths.
 
When you are selecting one midi track or the other, click on the Track Number - on the far left of the track in the track pane, and it should take care of it for you.  (if you had manually turned input echo on you might want to click it off first, to let Sonar handle it).
 
There is a setting in Sonar that tells it whether or not to automatically turn input echo on when bringing midi tracks into focus.  You might also want to check that to make sure it is set properly.  This setting is found in Preferences>Midi>Playback and Recording.
 
Bob Bone
 

Sounds to me like you've just got both MIDI tracks assigned to the same MIDI channel, or either one or both are set to omni mode.  Set SD3 to channel 10, the traditional channel for drums, and Dim Pro to channel 1. Or, if the SD3 track is done, select "None" as its input source.

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.

Thanks so much for the replies. Left work early, drove home and tried ALL of the above. I think I"m golden. Once again, Sonar user forum users come to the rescue!

For anyone who has a keyboard with a very easy way to change output channel on the front panel, it's pretty good to set each synth track to its own channel and have them all echo-enabled. Then pushing a button on the front panel changes to the synth track you want to play. Sometimes you can set up a few presets on a keyboard just for this purpose. Even better if the keyboard can send on multiple channels simultaneously, e.g. layers or splits.

bvideo
For anyone who has a keyboard with a very easy way to change output channel on the front panel, it's pretty good to set each synth track to its own channel and have them all echo-enabled. Then pushing a button on the front panel changes to the synth track you want to play. 

 
That's exactly what i do in live situations and it works like a charm.