It would be more than helpful to be able to copy-paste multiple citations into the “insert citation” box. For example, copy-pasting this: “Toju2014-ux, Mitchell1991-ce, Ashby2017-wm”. It is easy in PP to select several papers, copy all the keys using Ctrl-K but then I still need to insert them one by one in Google Docs!
At the moment you can use our sidebar add-on which allows you to search and add multiple citations from your library in one go.
We are testing a new citation dialog currently for our Word plugin and we have some very concrete plans to cite multiple items there. Eventually we want to bring all the new features we test in the Word plugin back to the Google Docs plugin.