Help setting up file containing both A415 (-1 st) and A440 instruments?

Hi all, I have a puzzle and would love some help setting up a file. I have a strings/wind/portative organ ensemble who will play at A415 which sounds a semitone lower than A440, but also a church organ at A440 that I’d like to have play with the A415 people. While I could just compose everything in one key as if it’s A440 and then manually transpose the church organ right before I hand in the piece, I’d feel better if there did happen to be a way to add custom transpositions (or per-instrument intonation systems?). I have the new 4.3 update.

You can use transposing instruments or use the transposition override feature in the Layouts right column in Setup mode… Seems pretty straightforward to me

Yes, write your complete score, as if all instruments would play in 415Hz.
When everything is finished do a Transposition Override for the church organ Layout (Written middle C sounds as: C#)