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I don't see why both must be done. It would be similair to replacing a cracked head. If one is bad, then only one needs to replaced. Just be sure that the other one is good and you don't have to change it sooner than later. About the only duplicate cost in doing it later is the wheel alignment.
You do not have to rebuild the arms in pairs.The question is are you rebuilding the arm or just installing the old spindle assembly on the new arm? If you have high mileage on an original set of bearings I would do both just so you know when they were rebuilt.
Of course the job involves money too, I'm setup to rebuild the arms so the part cost is low. If you send them out then you're looking at a lot more money. If they don't have a lot of miles, haven't been making any noise and they feel smooth by hand let them go. You can check the endplay to see what it is.
Good luck.
Gary