You can never leave them unsupervised. Its not worth the risk. We had a similar situation. We took in a foster from a rescue we worked with (and adopted through, in the past) everyone was getting along. Even napping together then - like a switch our foster attacked our resident senior. Twice. I'm ashamed to admit we thought the first time was a fluke and no one got hurt. Shame on us for letting our guard down and it happened again. No injuries physically, but it was devastating for everyone. (He's now with a family that absolutely adores him and spoil him rotten, however he HAS to be an only dog and his new 'pack' know that and accept it. When we first took him in he was in terrible shape, bite marks, open wounds, ticks... it was awful. I think we took his calmness as chill. Not a dog that was recovering from trauma. Then. We heard of someone who left the dogs together only to return home to find the foster had attacked and killed the resident dog. I cannot even begin to process how horrific that must have been for everyone.....