I want to compose a piece that’s polytonal with each instrument having a different key signature. How do I input independent key signatures in Dorico Pro?
To input a key signature for one staff only - click on the staff at the place where you want the key-signature to appear, press shift-K to bring up the key signature popover, enter the key signature, then press alt-Enter. If you press Enter without holding down the alt key, the key signature will be applied to all staves. Alt-Enter confines it to only the staff you clicked on.
Exactly as Steven says, just make sure that the thing you select before opening the key signatures popover is something that only belongs to one staff – if you select e.g. a barline that spans multiple staves, it won’t be possible to input a staff-specific key signature because Dorico won’t know which staff it should go on.
Selecting a note, rest, dynamic etc should all work as they are staff-attached.