My parents had littermates when I was younger. Tiggus and Crybaby never got into nothing more than a minor cat spat and dad was convinced they plotted things together lol. They both lived to be over 18 yrs old too! I think if two cats are going to get along, they'll get along and if they won't then they wont. I do think cats are social creatures and if they are with another cat from the start I think the odds are better that theyll get along.

Maybe they figure not adopting out littermates together is a way to prevent hoarding too