If you already have an interleaved stereo track, it will come into RPR as stereo, and can be plug-in-processed as such.
If the tracks are dual mono, they will come in on two separate mono tracks. In that case, I love the Folder methodology. You make a Parent Folder, name it Piano, and put stereo plug-ins on that Folder track. Then you can make the Child tracks (the originals) any size you want, or invisible if you want. The waveform will show up in the one visible Parent Folder track.
That's how I do it at least. I like it also because you can still change volumes individually on the children, or process only one in mono, etc., and the results still come to the Parent and can be processed together there.