Keeping them separate means keeping a barrier between them that they can’t bite each other through. Some people house their new rabbit next to their existing rabbit in a different cage. This allows them to get used to the site, sound and presence of each other without there being any potentially harmful interactions. She is probably becoming territoriality aggressive/defensive because she can smell the new rabbit, which she can probably also smell on you. For the time being, for feeding times and things I would feed your existing rabbit frist, before feeding the new one so that you don’t have the smell of the baby on you before feeding her. Given her lunging, it may be worth keeping the new one out of site for a week a week or so until Daisy settles down a little and isn’t lunging and acting so territorial. Once she’s a bit more settled, then bring the cage with the new little boy near where Daisy’s cage is for them to get used to being near each other. You won’t be able to introduce or bond them until you have had the boy neutered, but until then you can certainly use that time for them to get used to each other and preparing for bonding by scent swapping. This includes switching toys and food bowls between cages, you can also swap them between cages.