It would be really useful when harmonizing/counterpointing to be able to switch between Piano Roll views of two tracks with one click (or even to have both on screen at the same time). Is this possible in X3?
 
Cheers...

Yes. The PRV can display as many tracks as needed, use the Tracks menu in the PRV to select tracks to display. To open more than one instance of the PRV, right-click the existing PRV tab and lock contents. Then SONAR will allow another instance of the PRV. Also, in the track view, changing the edit filter to "notes" on a MIDI track will turn the track into an inline PRV.

Many Thanks and Happy New Year!
 

Once you have 2 PRVs open you can undock one or both and move them around so you can see the parts lined up side by side better.
 
 - Dave

You might also find Staff View useful. It can display several staves together.