Do I need a new layout if I want a player to play alto sax in 3 movements of a work and baritone sax in another? Can you change instruments between flows? How do I get Dorico to sound the new instrument without notation showing an octave too high or too low?
No need for multiple layouts. Write your music in Galley View, placing music in the desired staff (the player should be holding both alto sax and bari sax). The music will then display correctly on a per-flow basis.
Do I need a separate staff for each instrument? Hiding the one not needed for that flow?
No. Dorico handles instrument changes automatically in Page View (assuming you haven’t set Layout Options to disallow instrument changes, which is fairly unlikely given the question you’re asking). Two staves will show in Galley View; only one stave will show in Page View.
If you give one player 2 instruments, there will be 2 staves visible in Write Mode/Galley View. If you enter music in these staves (not concurrently af course), then Dorico will display 1 stave with automatic instrument changes in Page View. No need to hide things by yourself, Dorico does that for you.
Edit: Leo finished first.
Much thanks. I will try it!