Is it possible to view multi tracks in one single piano roll view?

Yes. One way is to select the tracks you wish to appear in the PRV and then open the PRV (ctrl - a) for all. They should be there. Another is open the PRV and use the track menu to select what tracks you want to display. There is also in the view menu the tracks view that lets you select the tracks or track to display.

Displaying MIDI tracks is also very versatile using the multidock.

In addition to displaying all tracks as John has said you can open as many PRVs as you want within the multidock with a combination of tracks displayed as you see fit. Right click on a PRV tab and select "Lock contents" to force a new one to open when double clicking a MIDI track.

Switch between them using ctrl+shift & arrow keys.

FastBikerBoy


Displaying MIDI tracks is also very versatile using the multidock.

In addition to displaying all tracks as John has said you can open as many PRVs as you want within the multidock with a combination of tracks displayed as you see fit. Right click on a PRV tab and select "Lock contents" to force a new one to open when double clicking a MIDI track.

Switch between them using ctrl+shift & arrow keys.

SEE, I didn't know that, and I would have never thought to ask. I've never been too happy about that...
So now I'm happy(er). Still, only about 11 tabs show on the dock while the rest hide behind the little arrows on the right.
 
It's posts like this that get saved in my Cakewalk/Knowledgebase folder for future reference ... cause I WILL forget!

In PRV I often display all, most, or whatever seems right, tracks. Then, using the Track Pane on the right, I can show or hide (or mute, solo, or edit enable or disable) any tracks. So, for example, if I have a bunch of tracks and there a "problem" with one or two, I can usually see it in the mess of notes, isolate just the tracks I want, and edit it from there. Then I have key bindings for Tracks/Show All, Hide All, and Invert. One of the best features in Sonar midi editing, IMO.