Calming/sleeping aid for cats

I have 2 cats, Cocoa and Marshmallow, they were "bonded" so we had to get them together. For the last few months they just fight, especially at night when we can't distract them. It wakes us up several times during the night, I imagine this is how new parents must feel (in a way). We've talked to vets and behaviouralists but literally nothing works. Would it be horrible of us to use "Naturvets Quiet Moments melatonin" on them? We're only getting about 3-4 hours of sleep because of their fighting and I'm scared their gonna really hurt each other. Also, surrendering one of them is not an option, we love and care for them both too much, AND we live in a 1 bedroom apartment so we can't really keep them separate