There is at least one service (CloudHQ) that will keep them in sync for a fee. What if your requirements are very lightweight, though?
In my case, I just want to be able to link a Google doc to a note. What I have been doing is:
- Open the doc in Google drive.
- Click the share button.
- Copy the link.
- Paste it in Evernote with Cmd-K, Cmd-V.
That's not a difficult workflow, but it isn't as light as it could be.
As an alternative, try doing a one-time install of the Google Drive app (if you don't see a button to install it at the link, that probably means its already installed on your machine). Then you can use Evernote's built-in "attach files" button or menu shortcut to put a Google Docs link right in your note. Just navigate to the right place in your local Google Drive folder and and click 'open'.