What you experienced with them is not uncommon. It doesn't have to do with them being apart for a month -- that is necessary after surgery.
It is simply that you are beginning to see their adult personalities. Almost all babies get along -- that is normal. BUT it does NOT mean that they are bonded. Baby "bonds" aren't considered true bonds because hormones can change all that. It is quite likely they would have had the same fights even if (or especially if) they had not been spayed.
That is a difficult concept for some to understand because baby bunnies can seem inseparable -- cuddling, grooming each other and seeming like the best of friends. It is natural to think they are bonded but it isn't a true bond.
Now that they are adults, it is as if they are two brand new bunnies. Bonding needs to be approached with that thought -- treat them as if they had never met before. (Indeed, they now have different odors/smells and seem totally different to each other.)
Since they had such a fight, it was a good idea to keep them apart for a bit to hopefully "forget" that they fought.
Just start slow by allowing their cages to be side-by-side, as you've been doing. The links offered by minirex will guide you through the process.
