You didn't provide any test results or diagnostic fault code numbers, so I'm just going by what has been real common and what your mechanic said is needed.
One thing I should try to explain as best I can is how these codes are set. Typically the signal from one sensor gets weak first, and you run to the mechanic. Based on the fault code number, and his experience, he knows to replace that wheel bearing. The problem arises when you try to live with this for many months. When the yellow warning light is on, the system is turned off and many of the self-tests are suspended. During that time the same problem may develop on the other front bearing, but since the tests are not being run, a fault code for that second bearing won't be set. When you finally do go in for service, all the mechanic has to go on is the one fault code stored in the computer's memory, so he writes you an estimate to replace just that one bearing. It isn't until he replaces the bearing, erases that fault code, and goes out on a test-drive that the self-tests resume and the missing signal from the other wheel is detected. If the problem is still intermittent, it may not be detected for minutes, hours or days. You might have the warning light turn on a few minutes after you leave the repair shop. Naturally you think the mechanic is incompetent or you were ripped off, but in reality, the car just needed more parts than either of you knew about. Mechanics hate this too because they know they did the proper repair but they also know this design makes them look incompetent.
When you turn around and go back to the shop, the mechanic will recheck his work and will start by reading the diagnostic fault codes again. That's where he will find a new code number for the other wheel. We hate having to tell you more parts are needed, but we have no choice. This seriously adds to the bad reputation we have that is mostly undeserved.
I got the impression you took the car in to be checked right after the warning light came on, so it's almost a certainty that only one wheel is causing the problem at this time, and you need just one bearing assembly. If that is correct, I suspect your mechanic told you both bearings are needed because he knows this is a real common problem, both bearings are the same age, (most likely), and have the same amount of play / wear, so to avoid hard feelings later, he knows it's best to just change them both right now. In the long run that is better than replacing one bearing now and making you come back a second time a month or two from now, when you won't be happy. Think of it like having a worn spark plug that causes a misfire. They're all the same age so even though only one is bad, we replace all of them at once. You'd really be unhappy if we made you come back numerous times every time one spark plug failed.
Tuesday, April 5th, 2016 AT 8:24 PM