What does adding "both" character sets do, exactly?

So I started with just simplified but now think it may be a good idea to learn how to read both at the same time. So I went into the settings and selected “both” as my character sets.

There’s not exactly an info label letting me know what that does, though.

I don’t really see much change except some SIMP label in the upper right corner when I review a card, every so often…

Will I have duplicate cards to review whenever I learn a new card that uses a simplified character and the cards learned so far are unaffected?

And if so, is there a way to set it to “learn to write just one, learn to read both”?

Yes, the cards learned so far are unaffected. You will need to go and learn the traditional versions of the characters that you have already learned if you want them to show up in your review. The Simplified and Traditional cards are separate entries in the system, so you will have all card types for both Simplified and Traditional (for the new entries that you study). At the moment, we do not have these type of micro settings. If you want to get very specific with what you study for Simplified/Traditional, you will have to manually swap between Simplified and Traditional on the main settings screen.

I have been learning both for the past four years. It makes the writing review take longer as you will have to review and write duplicates. Also, I noticed that in the other reviews (tone, pinyin/zhuyin, and definition) it seemed to prefer simplified over traditional when giving me cards. This may be because I started with simplified for my first year. The only duplications you’ll see are in writing. So if you are specifically wanting to review reading (pinyin/definition) in traditional, I would toggle it to traditional only when you go to review reading.