I often write/orchestrate using just a single external piano for playback of the full score (as to not get distracted by sampled sounds). Problem I noticed that under this setup, exporting a midi file combines all instruments into one. How do you get around this? Again, I don’t want to hear the internal sounds in Dorico while orchestrating.
Actually I just noticed that if I set every instrument in the sore to the same external piano on the same channel, but set each staff in Dorico to a different port, it then exports all instruments as separate midi tracks. And the external piano (CP4) plays all tracks with the same piano sound on channel 1 (does not have multiple ports as far as I know). I guess this could be a workaround, even though it seems it shouldn’t be necessary. Many composers/orchestrators don’t use sample libraries when actually writing or arranging the music since they’ve learned to hear the music in their head.
Hi @kmm,
EDIT: opss, I think I misinterpreted your request, sorry: I am sure you will become more help from someone else soon 
One thing you can try, though, is to just load an Orchestral Playback template just before exporting the MIDI, and don’t save your project.
(Here my previous suggestions):
I would suggest to load a Playback template with all your orchestral sounds, channels etc, assigned, and if you want to hear only a piano sound while inputting, deactivate MIDI thru, and use a standalone piano VST (on MacOS, for windows, is different, see links below).
So you have the advantages of piano sound by inputting, nice playback of the inputted notes to check, and correct MIDI export.
Links
Here the post with explanation:
Playback puzzle - #12 by FredGUnn
and/or this :
Different instrument sound in write mode? - #2 by Christian_R
Thanks for the reply. That may be an option as well. However, I don’t just want to hear the piano while inputting, but also upon playback. Occasionally I may use a sample or two, especially for longer sustained parts, but most of the time, I find it clearer to hear exactly how the harmonies and rhythms interact on a single instrument, as well as how real players would actually approach their parts as opposed to sampled players. If I do use sampled parts, I will usually add them in later during recording.
Would setting the Playback Template to Silent help you compose in silence?
Silence doesn’t work. All tracks will be merged together upon midi export into a single midi track. Any staves assigned to the same instrument/channel whether it be internal or external gets merged upon export.