It is quite something to consider, can it be good? Can it be destructive?
As someone who suffers from mental illness I can assure you at first I thought how on earth could be beneficial for either party if two mentally ill people were to try and help each other. Then I stumbled across this article by Fiona Thomas about exactly that. It sounds silly to others but when you think about it, what can help someone with a mental illness feel more comfortable than someone who knows exactly how they feel?
After contemplation, I started a blog about my journey with mental illness and I remember just hoping at least one person with mental illness feeling alone would read it and realize they aren't alone. After I got a few views and people started reaching out and opening up, I realized that the mentally ill can really help each other.
After posting on a few websites the founder of House of Misfits saw my post and invited me to join her server. The quick summary is it's a discord server with support channels for any individual needing advice, someone to vent to or help to de-escalate a situation. It is clearly stated that the listeners are not therapists, it is literally people with mental illnesses and experience with mental illness offering support to others. I am a listener myself and I never could have guessed that anything good could come from my depression and anxiety until I learned to share my coping methods to help others in need. Don't get me wrong I am not implying that therapy is not necessary as one of the most common questions we ask as listeners is whether or not the user has sought professional help but, not everyone can afford it and sometimes people just need a stranger to listen to how they are feeling.
Talking about mental health is a topic that gets avoided quite often, it shouldn't be because people shouldn't feel ashamed of having mental illnesses. Mental Health America has an article regarding talking about mental health that I just love, it is very insightful.
To conclude, I do believe the mentally ill can support and help each other.