I’m sorry you are having a rough start!

It is absolutely essential that she cannot reach him through the pen bars, so you will need to either add another fence to create a 6 inch buffer, or use small mesh panels to prevent the biting. It’s very normal for there to be a lot of aggression when not in neutral territory. The more biting you have across the fence, the harder things will be down the road.

I would recommend penning them both for now, in side by side pens. It will be important for Lily to stop viewing the whole space as her territory, which will take some time. You can start by adding a visual barrier between the pens as well, such as blanket or towel clipped to the pen wall. Don’t do any side swaps at first, and just give them a week or two to get settled into this arrangement. This will also help Lily’s stress levels because she will be able to get used to this new bunny in a more gradual way. You can alternate which bun gets out to exercise (I usually give one bun morning exercise and the other evening exercise when I’ve used this arrangement.

Gradually remove the visual barrier. Then once they seem OK with this set-up, start swapping which bun is in which pen every day or two. Continue this until Lily is not longer reacting strongly to Milo. Usually this takes a couple of weeks. It can really be a dramatic change from day 3 to week 2, so don’t despair just yet.

After all of this, once the buns are acting calmly around each other after the side swaps, you can start introductions in a very neutral territory. I would aim for a place that is somewhat stressful and do a very short session (set a timer for 2-3 min). Pet them a lot and do not allow any aggression. Continue that daily and start slowly increasing the amount of time if they are not immediately attacking. If you do get immediate attacks, I would try stressing with a car ride together, but don’t worry about that just yet.

I’ve had some very difficult bonds that ended up with VERY closely bonded bunnies that were inseparable, it usually just takes time and patience.

I think considering her reaction towards Milo it’s ok that they are on opposite sides of the room for now! Perhaps after a week or so she will settle a bit and you  then you can consider some rearranging.

Females can be very territorial. Rabbits also experience what’s called “referred aggression” where they smell or see a perceived threat and will attack whoever is closest to them. I do think she will calm in time! Some people have luck with getting a stuffy and swapping that between pens as it will have the other rabbit’s scent on it but won’t fight back. The best thing to help her calm down is time and a little reassurance. Aggression in rabbits is fear based, so she needs to learn he isn’t a threat.

My first pair was a tough bond. My original bun was very territorial and would attack the new bun on sight, but he did eventually (finally) come around and actually ended up being the submissive bun in the pair! So it can be very hard to predict and usually things do work out with enough time!

Did you happen to rescue Milo from a place that can provide bonding assistance? Sometimes it can be helpful to take the buns to the shelter and have an experienced bonder help with the first sessions.