It is common for academic articles to have a Supplementary Material section to provide evidence or discussion in support of the main argument that space constraints do not allow to be published with the article.  The Supplementary Materials in turn often have their own citations separate from the manuscript.  This post details how to create these two separate bibliographies.

Because Latex was created by a demi-god whose intelligence is beyond mere mortals like myself, this task is complicated.  To me, creating a bibliography at the end of the main paper and another at the end of the Supplementary Materials should put the citations in their correct places.  Instead, without further work, it creates the same bibliography in each location.  Because of course.

The key is to use the multibib package and put a \newcites{} thing in your preamble.  (What follows is essentially my translation, with a few more details, of this Overleaf help page  and this template.  In other news, I’m falling more and more in love with Overleaf: it brings Latex from 1978 to 2019.)  I added the following two lines to my preamble:

\usepackage{multibib}

\newcites{SM}{SM References}

Then, I include the bibliography in my main.tex file like normal.  Next, I add to my SM.tex file those same two lines, slightly modified.

\bibliographystyleSM{apsr}

\bibliographySM{library}

The key here is that the main manuscript does not have the
SM before {apsr} or {main}.  In addition, all citations must include SM as well, i.e. all \citep{} -> \citepSM{}.  Also, “SM References” becomes the title of the references page of the supplementary materials.

The next step gets a little tricky.  As anyone who has made the mistake of using Latex knows, after the first latex compile, you then have to do a bibtex compile followed by two more latex compiles.  (Why can the program not do this for you?  Because of course.  Overleaf, however, does this for you!)  After the first latex compile, two .aux files will be created, one for the main manuscript and another that is SM.aux.  You have to bibtex compile SM.aux on your own.

In my version of TexShop, this process does not work through the GUI, so you have to use Terminal.  bibtex /path/to/SM.aux does the trick.  Make sure you make your current directory the project directory.  Because Latex, I would get errors if using absolute paths.  After this step is complete, you can latex compile the manuscript file twice.  So, overall, it is latex compile, bibtex twice, then then latex compile two more times.

Actually, you do not have to compile precisely in that order.  However, the Supplementary Materials will not have its own bibliography until you bibtex compile SM.aux.

I have used the letters “SM” throughout this example, but you can use whatever you want.  Anywhere this example has “SM”, replace with the letters you have chosen.

Many thanks to Saad Gulzar, Michael Freedman, and Matthew Pietryka for their help.