Hi, I am a first-time cat owner. Two days ago, I adopted two kittens from different litters. Maybe I should have adopted siblings? Anyway, the lady at the rescue said it'd be fine. They're both about the same age although the calico is a little bigger. They are fine chilling near each other, eat side by side, and they both sleep on my full sized bed with me. But every time they make accidental physical contact or happen to encounter each other they hiss and swat, and it happens like every 10 min. Is there anything I can do to help them get along or should I just give them time?