Q: How should the authorship be mentioned on a publication in case of two co-first authors?
How should I write it in Japanese when there are co-first authors (2 first authors)?
Different journals and publishers have different ways to express co-first authorship. For example, PloS One journals have the logo with the word "Co" written on it that is added after the name of each of the co-first authors. If you click on the name of each co-author, you get a full explanation that says both authors contributed equally to the work. Here is an example of an article with two co-first authors that you can check.
In some international journals, the term "co-first author" is added in footnotes or in brackets after the name of each co-author.
You need to find out which system is followed by your target journal. You can do this by going through the author guidelines section of the journal website in detail. You can also check some of the articles published in previous issues of the journal to get an idea of the convention that is followed. However, if you are still in doubt, it is best to get a clarification from the journal editor.
You might also find the following articles interesting: