While you can only open one instance of the actual version of the software, you can have dozens of "work" spaces opened within the software. Duh, they are windows! All Windows software uses this since 1995? So they (the projects) can all be re-sized to fit any amount of screen space.
I often open 2 projects side by side to transfer tracks. Even though I have 2 monitors, I do this on my main screen. I only need to see the track name in the inspector and a tiny bit of the actual track.
When I record a live session I open 24 or so blank template projects and minimize them. They line up neatly along the bottom. This requires Sonar to be in "floating mode" so you can see the desktop at the bottom. A second monitor would also work. This way I have a fresh project ready to go in less than 10 seconds.
PS: I'm pretty sure you can have two complete Sonar's open side by side too. They would just have to be different versions. Not sure if they would both play at same time?? OOk.