I have a 1 year old cat that we have had since June. Yesterday we adopted a 2 year old. We let them briefly see eachother through the cat carrier and my one year old hissed a couple of times. We have been keeping the newcomer separate, but today we opened the door a crack so they could see eacother. There has been no hissing, but little "prowwl?" sounds from both and pawing at eachother through the door. Is this a good sign?