Conductors don't like scores that have single instrument staves. However, they also don't like scores with more than two instruments per stave. Combining two instruments is a difficult endeavor and it is not done by using two voices. There are a bunch of rules:
If the two instruments play the same notes and same articulations and phrasing, write the passage in a single voice and mark it "a2".
If only one instrument plays, write the passage in a single voice and mark it "1" or "2".
For all other cases, use two voices. However, if the rests are common, draw a single rest in its normal single-voice position.
When using two voices, if voice 1 and 2 have common dynamics or staff text, place the dynamics and text in the normal position. If the dynamics and/or text are not common, place the voice 1 dynamics/text on top, and the voice 2 dynamics on the bottom.
When starting a new system, repeat any "a2", "1" or "2" marking.
Avoid cluttering up the score with a lot of "a2", "1" and "2". If the music changes that rapidly, use two-voices.
If the two-voice notation is hard to read, separate the single-staff into two staves just for the length of that system.
Any good engraving reference will show you how its done. Just about any score from any reputable music publisher will have examples.
The trick here is that the whole point of having a notation program is to automate music production, so we want to be able to take two one-staff instruments and automatically combine them without losing the ability to generate the individual parts. This is a tough problem and I don't think PreSonus is going to focus on it. To be fair, other software struggles with this, too.
The a2-style notation with the ability to extract parts is critical to reducing the effort required to create an orchestral composition. But it's becoming clear (to me) that Notion is not targeted at professional composers. And the user base reflects this: at this moment, this feature has 9 votes—if people were actually using Notion to deliver scores to orchestras, I think there would be a lot more votes.