I open a project made last year and add a second violin to it.
It appears on the score but not on the mixer.
I restarted Dorico, and also restart the computer but it didn’t fix it.
A part of the project
https://dropover.cloud/e94700
Try this:
Go into Play mode.
In the Play menu, select Playback Template…
Choose Silence, Apply and Close
Repeat the above steps, but instead of Silence, choose the Playback Template you want - for me, that was HSSE+HSO (Pro).
Woah it worked 
Thank you very much.
I just wish it didn’t happen in the first place but now at least I can fix it.
One thing I noticed in the file you supplied, and I don’t know how this would have happened, is that when I opened a Mixer window, the Violin 2 did not appear there (as you mentioned, now that I have just re-read your original post), but it does show as a MIDI instrument.
Yes, I don’t know why the Violin 2 didn’t appear in the mixer, but after your fix, it appear properly.