A rabbit screaming is an extreme sign of distress, so immediate separation was absolutely the best call. What’s most likely happening is they are trying to establish who is the dominant between the two. Commonly, the female tends to be dominant, though that’s not always the case. Since they are both fixed (and my assumption is both recovered with no issues; please confirm!), it would be wise to start safe and go through a more formal bonding process.

Rabbit siblings often get along very well; however, that isn’t usually because they are siblings– it’s because they are babies. Before hormones kick in, baby rabbits get along with virtually any other rabbit. Rabbits do not acknowledge family units, with brothers and sisters mating and fighting with each other being commonplace. Since they have both been fixed and have reached the points where hormones have developed, their relationship is most likely getting rocky as they need to establish a permanent bond, as opposed to the temporary “baby bond”. Every pair of rabbits need to establish an acceptance of who is dominant.

You can read about the basics of bunny bonding here: https://binkybunny.com/infocategory/bonding/

…. Since they seem to get on pretty well with exception of this major tiff, it is definitely important to supervise them whenever they are together. At times when you can’t supervise (i.e. when you’re sleeping), it may be safe to house them separately, though that is my opinion.

If you find their relationship continues to falter, you’ll want to go back to the basics of bonding, as if they never knew each other and house them separately. There’s a good chance they have some sort of a foundation if they’ve been with each other for a month with you, but that’s not certain. You will want to monitor and be open to backing down on their time together if you notice they cannot work out the hierarchy safely.

Agree with Wick 100%. Their hormonal changes after spay/neuter likely triggered the fighting, which is very common when rabbits are together as babies (check out the pinned post  “Why are my rabbits fighting!?”). This can be due to puberty hitting, or hormonal fluctuations post-castration. As Wick mentioned, babies don’t form true bonds, and these fluctuations can happen again, so you should separate them and then go through the bonding process once their hormones have settled further.