Unison Between Two Identical Instruments

Having two identical instruments on the same note can be a very delicate matter and there are a few orchestration guidelines that you should at least know about.

The problem with this is that with two identical instruments, often small differences in intonation will be way more obvious and exposed than with a solo player. If you add a third instrument to it, the intonation differences will start to even out and you will get more of an ensemble sound.

Interestingly, this phenomenon is very depending on the instrumental colour. While it is practically no issue on brass instruments (as long as they intonate halfway decently) the most problematic instruments with that are oboes and violins. 

In string quartet writing, there is a general guide to avoid having both violins on the same note as it will always sound weirdly out of tune, no matter how well the players intonate. This problem increases the higher you go.

If you have excellent players available and the line is not too high or too exposed it might actually not be too problematic to have both players on the same note. However as mentioned before this is really depending on the context and players. So if you want to be safe, it might be a good idea to follow this guideline.

Oboes a2 are in general not the greatest idea. If they are part of a big woodwind tutti (e.g. all woodwinds doing runs or figures), it is no problem to use them in pairs. However, as soon as the line is more or less exposed, avoid having two players on it. The intonation differences as well as slight differences in the strcuture of the harmonics will make the note or line feel pretty out of tune in spite of it being in tune. There are examples in the literature that don't follow this but results can be a hit and miss with these.

Generally with woodwinds, it is more attractive to have exposed lines on solo instruments. Doubling will create a thicker and less brilliant sound with woodwinds.