Howdy Claire Coffey,
Thanks for taking the time to reach out in Apple Support Communities. I understand you'd like to add a second application of Google Drive, known as Backup and Sync for the Mac, in the sidebar of Finder.
Having the option to customize your Mac is an important aspect to completing your daily tasks. I'll be happy to help and point you in the right direction.
You have the ability to add a second application of, Backup and Sync in the sidebar of Finder. I was able to confirm by testing this on my Mac. All you have to do is rename the second application and make sure it does not match the original app. For example, I have "Backup and Sync from Google" and the second application is named, "Backup and Sync from Google 2."
Once that's done, you can open Finder, select the second application you'd like to add in sidebar > navigate to the menu bar and go to, "File" > Add to Sidebar.
Please let me know if that helps.
Kind regards.