How to export 2 parts together

It's hard to understand what your goal is. Here are the conceivable variations for two clarinets:

• Do you want clarinet 1 and clarinet 2 on one staff? So with two voices on one staff?
• Do you want a clarinet extract with two staves, one for clarinet 1 and one for clarinet 2?
• Or do you want clarinet 1 on the top half of the sheet and clarinet 2 on the bottom half?

At the first point you have to merge the two parts with 'Implode'. You will then get a staff with two voices.
The second point is to create an extract with two instruments. You will get a sheet with two instruments.
For the third point, you need to save the two parts separately and then copy them together - with a section change in between.

You have 12 parts. Say you want flute and violin together to show as a part named "Flute and Violin".
For MuseScore 4, see:
https://musescore.org/en/handbook/4/parts#create-a-new-part
at step 3 give the name "Flute and Violin".
and after step 4 (when you open the new blank part) simply add your instruments (flute and violin) to the new part (so no longer blank),