Getting started with my stage 2

[analogika]

Ch 1 is left, basically everywhere.

[MrSitcom]

Can anybody tell me which channel is right, and which channel is left, when using Ch1 and Ch2 to go into a stereo amp?

I'd say that Ch1 is Left.

[Mr_-G-]

Perhaps there is a way to unambiguously resolve this: if the piano samples represent a stereo image from the player's point of reference, when you put the headphones in the 'correct way', the left hand keys should sound more prominent on the left ear of the player (and vice versa for the right side). You can even use the auto pan effect to make this more obvious.
But as Pablo rightly said, the listener might never expect a particular handedness of a stereo sound, so unles you are exploiting a particular spatial image it might not be too relevant. I would be useful to know if let's say you want the listener to hear the same as a the player would, or synchronise to something happening on one side of the stage or when creating some special sample.

[Adams]

Do which ever sounds more comfortable to how you monitor it. I use IEMs and channel 1 is panned left. Like Pablo says, it doesn't really matter. There's a setting on my Kronos to swap the piano sounds from player or audience perspective...

If you are splitting channels with different sounds, then it's 2x mono signals, not stereo, surely? Matters even less, really...