I am going thru the same issue right now and don’t know what to do. We have been isolating them , letting them into the house separately, but they still meet up and have hissing matches, but it is just posturing. We hope they will integrate soon. It is stressful for all concerned. The main thing is to give them separate litter boxes and separate eating quarters. Also, cats need to “own” an area so there,s that too.. You just have to give them their own time.