Woodwinds usually sound more brillant and characteristic when they’re playing solistically instead of doubling the same instruments in unison. For example 3 flutes playing a line in unison in generally sound thicker and slightly louder than a solo flute but lose their brilliance and airyness of the sound. This applies to practically every woodwind instrument when being doubled by the same instrument.
So as soon as a woodwind line is exposed, it usually is preferable to have it play solo instead of a2/a3. Another advantage of a solo is that you usually get a more expressive performance, as the player can take some freedom in the expression rather than needing to blend with his/her colleagues and playing more restrained.
This doesn’t apply of course when the woodwinds double other instruments or are part of a harmonic texture etc. (e.g. flutes doubling first violins) where it is common to double the line a2 or a3, often just for pure substance.
Also note that brass instruments behave differently. When you double the same instruments there, you will get a broader more substantial and majestic sound.