I'm doing an instrumental solo with piano accompaniment and the two instruments are in different keys. When doing the solo outline + accompaniment, is there a way to mark this, or is it not necessary because it was noted in the full version of the solo part?
I would place the key signature for each part after each marginal hand sign (and after a space) as we would if there were a key change or a time signature change. I have had some Bartok piano pieces in which the two hands are in different keys. This is how I indicated those different signatures. The time signature would remain in the music heading with the tempo indication (if present), but without a key attached to it. You can always cover your bases by adding a sentence about the irregularity on the Transcriber's Notes page or right before the music to give the reader a heads-up.