advice from owners of two

[YoureAllABunchOfBastards]

Have only ever had single cats before.

Big Ginge is Little Ginge's dad. They rough and tumble a lot and Big is definitely dominant. This morning, though, he has hissed very nastily at Little Ginge a few times and Little is looking terrified.

Do they just do this sometimes?

I don't know about the looking terrified bit but in terms of rough and tumble ours do that a lot. It looks and sounds quite nasty but actually there is no real bites or scratches. We had a 5 year old neutered female when our tiny 6 week old female arrived. To begin with she mother Ed her but when the kitten reached about 4 months all bets we're off. Older cat is still the dominant one and will hiss at the kitten when she has had enough or to tell her she's not in the mood. Despite all this they share a food and water bowl and a litter tray despite us trying to give each their own as is recommended so they must like each other a bit. Are they both boys? You will probably need to get big ginger 'done' if you want them to get on (obviously even more important if little ginge is a girl)

Both boys and both done.

Just seems to have a nastier edge this morning. I have told them both to play nicely

Ours do this sometimes. The younger one loves to annoy the eldest but sometimes she's just "not in the mood".
It could also be that the little one is getting big enough to not be as submissive - so Dad has to put him back in his place.

Our two (siblings) sometimes hiss at each other, never really worried about it and just made sure they have space to give each other soon room if required. We have occasionally separated them for a bit if really nasty (only ever been when it was transferred aggression as another cat in their garden).

I think they just have to establish rules, and as long as no one is getting injured/moving out, it's fine. I've had my current boy cat with three different females sequentially (my old cat who then died, a rescue we got who only lived a few years due to a heart murmur, and now our new female rescue). In each case, the girls have been the boss, and would hiss and spit at my boy at times, often sounding quite savage - though at other times you'd find them curled up close together. He's never seemed upset, but clearly knows not to mess with them, if that makes sense?
It's funny, as my boy is relatively dominant around the neighbourhood (and will stand up to much bigger cats than himself!) but at home he's a total sub!