If you have a significant hearing loss in both ears, you should be wearing two hearing aids. Here are the audiological reasons why:
Localisation. The brain decodes information from both ears and compares and contrasts them. By analysing the miniscule time delays as well as the difference in loudness of each sound reaching the ears, the person is able to accurately locate a sound source. Simply put, if you have better hearing on one side than the other, you can't accurately tell what direction sounds are coming from.
Less amplification required. A phenomena known as “binaural summation” means that the hearing aids can be set at a lower and more natural volume setting than than if you wore only one hearing aid.
Head shadow effect. High frequencies, the part of your hearing that gives clarity and meaning to speech sounds, cannot bend around your head. Only low frequencies can. Therefore if someone is talking on your unaided side you are likely to hear that they are speaking, but be unable to tell what they have said.
Noise reduction. The brain has it’s own built in noise reduction which is only really effective when it is receiving information from both ears. If only one ear is aided, even with the best hearing aid in the world, it will be difficult for you to hear in background noise as your brain is trying to retain all of the sounds (including background noise) rather than filtering it out.
Sound quality. We are designed to hear in stereo. Only hearing from one side sounds a lot less natural to us.