I've actually done this as well, but it's still not super satisfying. There's still no unified inbox so I have to have two tabs open in the Fluid app, and I don't get notifications or any nice native features like that. On that same grain, Boxy[1] is an unofficial Mac app wrapper for Inbox that adds notification support, but is even worse because it requires me to have a window open for each Gmail account (no tabbed interface).