Two hearing aids are usually best, and the most intuitive choice most of the time.

Spatial hearing matters, and using two hearing aids is generally the best way to achieve this if you have any hearing damage.

In complex situations, where lots of different sounds are competing for our attention, some sounds mask other sounds. There are many reasons why this situation is particularly hard for people with hearing loss, but appropriate binaural hearing aids will help.

I must stress that hearing aid characteristics vary considerably between brand, model and style, and this has caused some inconsistencies in the research on the topic of how much help binaurally fitted hearing aids give.
Localisation is important for orienting to sound, and hence attending to it. The brain uses different cues to localise low pitch or high pitch sounds and this means that a disruption in hearing acuity may make some sounds easier to locate than others. In order to use any of the cues, you must be able to hear them, so some hearing aids help some people to a lesser or greater degree.

You put yourself at a disadvantage if you are only hearing on one side, because sounds on the poorer side may be missed and sounds in the middle will appear to be displaced towards the aided ear. Binaural hearing allows us to select the most important of two signals.

Overall, wearing two hearing aids rather than one, makes selective listening easier, so that you can focus on the conversation you want to hear.

The hearing system combines the inputs from two ears, so that, if you use two hearing aids, then you don’t need the volume so high in either of them. This phenomenon is called binaural summation. If the hearing aids are not being pushed so loud, they might even sound better and are less likely to whistle.
Some aspects of hearing and discrimination deteriorate if they are not used, so two hearing aids will keep both ears active, not just one.

Research has shown that when only one hearing aid is worn, the unaided ear tends to lose its ability to hear and understand. Research with more than 5,000 consumers with hearing loss in both ears, by Sergei Kotchkin (Binaural Hearing Aids: The Fitting of Choice for Bilateral Loss Subjects. Knowles Electronics: Itasca, Ill, 2000 )demonstrated that binaurally fit subjects are more satisfied than people fit with one hearing aid.

I usually recommend two hearing aids, even if the loss in the better ear is really mild, as the hearing aid user has more control over the balance of their hearing and is not disadvantaged in situations where the unaided ear is in the “noisy spot” or the “dangerous spot, such as having a car passenger beside you as you drive.