Hi!
It should work the way rbowser described. Save and close the project, then try it in a new project. Sometimes a project can get a bit corrupted.
To get the job done if the split mono bounce doesn't work, then create two sends one panned 100% left and another 100% right to two new mono busses.
I haven't used Sonar in a while, you'll need to bounce each buss to make a permanent copy of each wave form, then drag them into two new mono tracks. I have forgotten how this is done from Busses. It might be a mixdown of each buss. Perhaps solo each buss then mix down the selected bus.
Now listen to some one else...
I tried.