Hi, I have to write a piece for two differents instrumentations. In my case, the same piece should be playable by a typical english brass band, and by a concert band. Some of the parts are exactly same in both (at least the percussion, but maybe trombones and horns).
How do would you do? One file, how could it be organized/configured? Or simply two differents files, and a lot of copy/paste?
Thank you for your ideas and advices 
You can keep everything in one file for sure. Use Player Groups to organise the different ensembles and influence the auto-numbering of instruments. You can make two different score layouts for the two setups. I’d also advise you to make a third, ‘working’ layout containing all the players, to make sure both versions stay consistent with each other. You probably need a big screen for that, though.
My spontaneous thought would be to have separate files for the two versions, but as hrnbouma points out, it could be useful to have a combined score too. Particularly if you want to make sure the two editions are the same so they can be used for a “mass band” together.
So I agree that one big file, while a bit unwieldy with a lot of players and layouts might be useful.
I wouldn’t work on both at the same time though. I’d write for one band and then transcribe it for the other as I wouldn’t want to risk treating the solo cornets as a bad clarinet substitute or vice versa. I’d finish it for one band before either adding the other players to the project.
But I’ve never written for two bands at once, so I’m not speaking from experience. (I’ve written for concert band, and played a lot of brass band but never written for one.)
I tend to combine alternate versions into a single Dorico file, just as one would want to use the same file to generate an orchestral and a piano/vocal rehearsal version of a choral work.
One key is @hrnbouma’s recommendation to have a never-printed “workfile” score layout, not just to make sure the versions stay consistent, but also as a safety to make sure removing an instrument from one version does not erase the notes entirely.