You need to let bun number 2 settle in before you even attempt bonding. In fact you need to let him settle in, then PRE BOND before an actual physical introduction.

As they’ve fought/she’s terrified, you need to put them in separate rooms so they can forget each other.

Sarah nailed it. They need to be in completely separated rooms now so they cannot see or smell one another. Wash your hands inbetween handling buns to help mask scent. You need your girl to completely forget he exists, and visa versa. This could take up to three months. I would wait the full three given that skin has been broken. Once the three months is done, you can keep their cages next to one another for a month, to help them settle, and then after that month (now 4 months from now) you start prebonding. For prebonding, youll swap their toys, litter boxes, bowls, ect and let them mark everything. Once a day youll do the swap. Youll do this prebonding for atleast a month. After that month (5 months from now), you can start actual bonding in a neutral territory, while continuing with prebonding.

If she is forever scared of him or other bunnies, she will never be able to bond with another rabbit. Are you prepared to have two single buns who cannot be anywhere near one another, ever, so long as they are alive? If not, time to start making back up plans, just in case.

Not all rabbits like to have others around. If you and your husband give her enough attention and grooming, she will be a happy single bun.

I agree with Sarah, separate and let new bun make himself at home in his new environment. Rabbits are notoriously non-easy going when it comes to meeting other rabbits. They have a very strong sense of territory. In nature, that works because there’s lots of space and a bun that feels threatened can simply run away.

Some bonds are difficult and require lots of work. Some rabbits won’t bond. I hope yours will. For now, separate.

I think coexisting would be good. Rabbits benefit from having another rabbit around. Then you can start switching toys or blankets between them, then swop litterboxes, then do proper bonding.

Lunging at the bars when inside the cage is typical territorial behavior, esp from a girl. Many girls absolutely hate it when somebunny or somebody messes with their home. Just make sure they can’t reach through the bars and bite, they can do serious injury to eyes and face.

I’d too trust a rescue charity, but one method doesn’t always work with all rabbits. Some rabbits take to each other right away, they’re an instant match. But many rabbits don’t, and for them, you have to figure out another method.

You really need them to forget one another and keep them away from one another. They will continue to act defensively and scared until they forget one another exists. (Yes, lunging is aggressive and is often done because of territorial drive, or fear) If you dont have different rooms to put them in, set them up on opposite sides and completely block of their cages and line of sight (suggestion: xpens with blankets over them). This way, when one is free roaming, they cannot get to the other bunny at all and cannot see one another. Again, youll need to do this for around 2 to 3 months. As long as they remember one another, theyll continue to act in fear when they notice the other. Fearful rabbits will not bond.

Once you think it is safe to share space again, introduce them in a neutral space like a bathtub or closet they never go in otherwise. The territorial-ness is quite strong for some rabbits (or maybe all rabbits) and the one-is-caged one-is-not has not gone well for my buns. Whoever is caged feels threatened because they cannot escape.

As for separating them in a small apartment, place a piece of furniture between them, like a couch, or keep one in the bathroom. A small chair or stool with a blanket over it can also serve as a visual barrier.