@dan_krieder gets the credit for teaching me this, but it is so helpful, I wanted to expand on it a bit.
Because of a MIDI import, I had a lot of winds wrong in the score: two flutes with the stems going the same way, for example.
Here is a picture version of Dan’s excellent and time-saving tip.
Select all the notes.
Go to Edit | Filter | Notes in Chords to select the top or bottom notes.
Right click any of the highlighted notes and select Change voice.
In my case, I selected Down-stem Voice 1.
In Dorico Pro 4, once one has selected just the bottom notes, one should also be able just to press V (or SHIFT + V if no downstem voice had already been created) to effect the voice switch.
If they are genuinely two different Flute parts, then ideally you should separate them onto two separate Player staves and then use Condensing to display them together in the score. Then you can have them as separate Parts.
Thanks, benwiggy. I need to study up on the Condensing feature. I sort of dove into Dorico with a full orchestral piece. Thanks to Dorico’s good design and this forum, I’m learning pretty fast. (Unlike Finale. Ugh.)