Understanding how “a2” works

"a 2" is rare on instruments that can play more than one note at a time and is normally added to a single note on a beat.

As an example, if 2 flutes are notated on 1 line and there are two notes on a beat, it is clear that flute 1 plays the higher note and flute 2 plays the lower note unless the stems indicate otherwise. When there is only one note on a beat, or more commonly several beats in a row, "a 2" is used to indicate both instruments play together. On my flute example, when there is a return to 2 notes on a beat, it is obvious that they are to once again play their own notes and no further notation is necessary.

I have rarely seen this on instruments capable of playing multiple notes like the violin, but when I do it is normally preceded by a divisi section. From what I can tell, this informs that violins that they are to play double stops (chords) rather than divisi. It is the same as writing unison except for when I've seen this. When I've seen this, there is a divisi into several parts and there are two violin staves. The top staff is being played divisi by some violin I's and the other staff is being played divisi by other violin I's. The "a 2" is applied to each staff. Unison is reserved for the point where all of the violins or their sections are playing the same notes again, even though there may be two staves visible to the end of the system.

This is the theory behind the notation. In the case of violins, the reality is that the concert master (lead violinist) determines which violins play which notes.