Villagers will not learn a new skill on their own. If you want them to work on something different, you should train them in that skill. Then, if you let them decide what to work on, they will usually choose to do one or the other of those skills, but not necessarily equally. If you want them to work in one skill area, you should set that as their preferred skill. They may still choose to work on both skills, but will select the preferred skill far more often.
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Barbara
Unicorn
Last Day of Work