Since one bun was seriously hurt, you’ll want to take a long break (several weeks), without pre-bonding, to let the buns forget about each other for a while. The exact number is hard to know, I’ve heard anywhere from 6 weeks to 3 months. Who knows how long a bun’s memory is though? After that resume pre-bonding as other’s have said.
It’s not a lost cause, but it may make things very challenging. The cases where one bun immediately attacks the other are pretty rare… where you in neutral territory when this happened?
Once you resume (if you decide to try again), I would opt for a VERY large, very neutral, and somewhat stressful location. Like 2 pens set up in a neighbor’s backyard (weather permitting). You’ll want to provide space for them to move (so they can’t immediately latch on to each other) and stress (so they realize there are more important things to worry about). If they still immediately attack in that type of environment they may just be one of those pairs that won’t work out.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.