Using Gigaplayer, I can stack two instruments in one track, and while all editing is not available once you stack them, some basic adjustments, including volume, can be made.
As an example, I can load a violin section and a violin solo to one track, and adjust the volume of the solo violins down, not to overwhelm the violin section.
Trying to do this with Aria, I can set the two instrument to the same MIDI channel, setting the output for both at 3/4, set Sonar's track to the same MIDI channel and have both sound play fine on one track. But, of course, the volume of both will be the same and I see no way to adjust the two volumes differently.
What is the solution? Have two separate tracks? Have the data on these two tracks the same? Would that be a lot of space and effort wasted?
You have mentioned somewhere before, that combining section sounds with solo is a good practice. How you do this?