One bun being affected doesn't necessarily mean the other bun will be affected at the same rate, if it is mange. Though both should certainly be treated even if one bun isn't showing signs yet. It's possible your one bun was more susceptible due to just having surgery and it weakened her immune system.
If this is mange and your vet prescribes the spot on Revolution/Stronghold (selamectin), because it's applied topically, your rabbits will likely need to be kept separate for I think 24 hours (package instructions should give info), so they don’t try and groom it off each other. There's a possibility that a temporary separation could affect their bond. So this might make using ivermectin instead, less hassle as it can be given as injection or orally, depending on what your vet prescribes. Then your rabbits wouldn't need to be separated.
In case the vet didn't mention this or you aren't aware, both rabbits should be brought to the vet together. Not only because they're a bonded pair and it's best not to separate them as it can affect the bond, but also because if mange is found on your one bun, the vet may also want to check your other rabbit.
Though mange certainly isn't something anyone would want their rabbit to have, in this case I do hope this is all your rabbit has and it isn't the alternative. Mange, though a pain to deal with, is treatable.
