I have had lots of 2's, 3's, and 4's...cats and dogs...from the same litter without serious issues. I have also fostered/adopted kittens and had the kitten bond with an existing older, male cat with them being almost inseparable, too.
I had 2 red (yellow-colored) males who were very close...my "Butterscotch Twins" and when one passed at 13 months old, the other was as distraught as I was over the loss of my little heartbeat kitty. Although he passed, too, a few weeks ago at almost 14 years old, I think he missed his brother his entire life. I still have 2 of that litter of 4, but the only female has not been close to her brothers like her brothers were close to each other since they quit the kitten-hood playing.
Brody and Mia are 6 days apart. Brody was 14 months old when we got him. Three months later, we took Brody to see if he got along with Mia, who is 5 days older than Brody but both were 17 months old by then. They were great friends almost immediately and remain so.
I think it depends on the animals themselves, and how the owners handle them might make a difference also. If you are considering getting 2 who are already close, you will probably not have serious issues. If you get 2 who already compete or fight, maybe not a great idea.
