Good evening Steinberg Community, how can I write a simple division (a 2) on the same line of an instrument, instead of separating it in two different lines? Thank you 
Two things: you can simply write “div.,” which indicates that dyads (even with shared stems) are to be played by divided players, rather than double stops (for strings, that is), or you can write “a 2” for something like two horns playing in unison. Also, you can use a new Downstem Voice 1 to differentiate the second voice.
If you want to have a division that appears on separate staves in the part but on a single staff in the score, as of Dorico 3.5 you can also use the divisi feature to split the division onto as many staves as required, then condense them in the full score.